From a96d684ffab11d6f40a8f1dde3c8103ab1d2bd27 Mon Sep 17 00:00:00 2001
From: Kohya S <ykumeykume@gmail.com>
Date: Tue, 15 Jul 2025 20:44:43 +0900
Subject: [PATCH] feat: add Chroma model implementation

---
 library/chroma_models.py | 706 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 706 insertions(+)
 create mode 100644 library/chroma_models.py

diff --git a/library/chroma_models.py b/library/chroma_models.py
new file mode 100644
index 00000000..9f21afad
--- /dev/null
+++ b/library/chroma_models.py
@@ -0,0 +1,706 @@
+# copy from the official repo: https://github.com/lodestone-rock/flow/blob/master/src/models/chroma/model.py
+# and modified
+# licensed under Apache License 2.0
+
+import math
+from dataclasses import dataclass
+
+import torch
+from einops import rearrange
+from torch import Tensor, nn
+import torch.nn.functional as F
+import torch.utils.checkpoint as ckpt
+
+from .flux_models import (
+    attention,
+    rope,
+    apply_rope,
+    EmbedND,
+    timestep_embedding,
+    MLPEmbedder,
+    RMSNorm,
+    QKNorm,
+    SelfAttention
+)
+from . import custom_offloading_utils
+
+
+def distribute_modulations(tensor: torch.Tensor, depth_single_blocks, depth_double_blocks):
+    """
+    Distributes slices of the tensor into the block_dict as ModulationOut objects.
+
+    Args:
+        tensor (torch.Tensor): Input tensor with shape [batch_size, vectors, dim].
+    """
+    batch_size, vectors, dim = tensor.shape
+
+    block_dict = {}
+
+    # HARD CODED VALUES! lookup table for the generated vectors
+    # TODO: move this into chroma config!
+    # Add 38 single mod blocks
+    for i in range(depth_single_blocks):
+        key = f"single_blocks.{i}.modulation.lin"
+        block_dict[key] = None
+
+    # Add 19 image double blocks
+    for i in range(depth_double_blocks):
+        key = f"double_blocks.{i}.img_mod.lin"
+        block_dict[key] = None
+
+    # Add 19 text double blocks
+    for i in range(depth_double_blocks):
+        key = f"double_blocks.{i}.txt_mod.lin"
+        block_dict[key] = None
+
+    # Add the final layer
+    block_dict["final_layer.adaLN_modulation.1"] = None
+    # 6.2b version
+    # block_dict["lite_double_blocks.4.img_mod.lin"] = None
+    # block_dict["lite_double_blocks.4.txt_mod.lin"] = None
+
+    idx = 0  # Index to keep track of the vector slices
+
+    for key in block_dict.keys():
+        if "single_blocks" in key:
+            # Single block: 1 ModulationOut
+            block_dict[key] = ModulationOut(
+                shift=tensor[:, idx : idx + 1, :],
+                scale=tensor[:, idx + 1 : idx + 2, :],
+                gate=tensor[:, idx + 2 : idx + 3, :],
+            )
+            idx += 3  # Advance by 3 vectors
+
+        elif "img_mod" in key:
+            # Double block: List of 2 ModulationOut
+            double_block = []
+            for _ in range(2):  # Create 2 ModulationOut objects
+                double_block.append(
+                    ModulationOut(
+                        shift=tensor[:, idx : idx + 1, :],
+                        scale=tensor[:, idx + 1 : idx + 2, :],
+                        gate=tensor[:, idx + 2 : idx + 3, :],
+                    )
+                )
+                idx += 3  # Advance by 3 vectors per ModulationOut
+            block_dict[key] = double_block
+
+        elif "txt_mod" in key:
+            # Double block: List of 2 ModulationOut
+            double_block = []
+            for _ in range(2):  # Create 2 ModulationOut objects
+                double_block.append(
+                    ModulationOut(
+                        shift=tensor[:, idx : idx + 1, :],
+                        scale=tensor[:, idx + 1 : idx + 2, :],
+                        gate=tensor[:, idx + 2 : idx + 3, :],
+                    )
+                )
+                idx += 3  # Advance by 3 vectors per ModulationOut
+            block_dict[key] = double_block
+
+        elif "final_layer" in key:
+            # Final layer: 1 ModulationOut
+            block_dict[key] = [
+                tensor[:, idx : idx + 1, :],
+                tensor[:, idx + 1 : idx + 2, :],
+            ]
+            idx += 2  # Advance by 3 vectors
+
+    return block_dict
+
+
+class Approximator(nn.Module):
+    def __init__(self, in_dim: int, out_dim: int, hidden_dim: int, n_layers=4):
+        super().__init__()
+        self.in_proj = nn.Linear(in_dim, hidden_dim, bias=True)
+        self.layers = nn.ModuleList([MLPEmbedder(hidden_dim, hidden_dim) for x in range(n_layers)])
+        self.norms = nn.ModuleList([RMSNorm(hidden_dim) for x in range(n_layers)])
+        self.out_proj = nn.Linear(hidden_dim, out_dim)
+
+    @property
+    def device(self):
+        # Get the device of the module (assumes all parameters are on the same device)
+        return next(self.parameters()).device
+
+    def enable_gradient_checkpointing(self):
+        for layer in self.layers:
+            layer.enable_gradient_checkpointing()
+
+    def disable_gradient_checkpointing(self):
+        for layer in self.layers:
+            layer.disable_gradient_checkpointing()
+
+    def forward(self, x: Tensor) -> Tensor:
+        x = self.in_proj(x)
+
+        for layer, norms in zip(self.layers, self.norms):
+            x = x + layer(norms(x))
+
+        x = self.out_proj(x)
+
+        return x
+
+
+@dataclass
+class ModulationOut:
+    shift: Tensor
+    scale: Tensor
+    gate: Tensor
+
+
+def _modulation_shift_scale_fn(x, scale, shift):
+    return (1 + scale) * x + shift
+
+
+def _modulation_gate_fn(x, gate, gate_params):
+    return x + gate * gate_params
+
+
+class DoubleStreamBlock(nn.Module):
+    def __init__(
+        self,
+        hidden_size: int,
+        num_heads: int,
+        mlp_ratio: float,
+        qkv_bias: bool = False,
+    ):
+        super().__init__()
+
+        mlp_hidden_dim = int(hidden_size * mlp_ratio)
+        self.num_heads = num_heads
+        self.hidden_size = hidden_size
+        self.img_norm1 = nn.LayerNorm(hidden_size, elementwise_affine=False, eps=1e-6)
+        self.img_attn = SelfAttention(
+            dim=hidden_size,
+            num_heads=num_heads,
+            qkv_bias=qkv_bias,
+        )
+
+        self.img_norm2 = nn.LayerNorm(hidden_size, elementwise_affine=False, eps=1e-6)
+        self.img_mlp = nn.Sequential(
+            nn.Linear(hidden_size, mlp_hidden_dim, bias=True),
+            nn.GELU(approximate="tanh"),
+            nn.Linear(mlp_hidden_dim, hidden_size, bias=True),
+        )
+
+        self.txt_norm1 = nn.LayerNorm(hidden_size, elementwise_affine=False, eps=1e-6)
+        self.txt_attn = SelfAttention(
+            dim=hidden_size,
+            num_heads=num_heads,
+            qkv_bias=qkv_bias,
+        )
+
+        self.txt_norm2 = nn.LayerNorm(hidden_size, elementwise_affine=False, eps=1e-6)
+        self.txt_mlp = nn.Sequential(
+            nn.Linear(hidden_size, mlp_hidden_dim, bias=True),
+            nn.GELU(approximate="tanh"),
+            nn.Linear(mlp_hidden_dim, hidden_size, bias=True),
+        )
+
+        self.gradient_checkpointing = False
+
+    @property
+    def device(self):
+        # Get the device of the module (assumes all parameters are on the same device)
+        return next(self.parameters()).device
+
+    def modulation_shift_scale_fn(self, x, scale, shift):
+        return _modulation_shift_scale_fn(x, scale, shift)
+
+    def modulation_gate_fn(self, x, gate, gate_params):
+        return _modulation_gate_fn(x, gate, gate_params)
+
+    def enable_gradient_checkpointing(self):
+        self.gradient_checkpointing = True
+
+    def disable_gradient_checkpointing(self):
+        self.gradient_checkpointing = False
+
+    def _forward(
+        self,
+        img: Tensor,
+        txt: Tensor,
+        pe: Tensor,
+        distill_vec: list[ModulationOut],
+        mask: Tensor,
+    ) -> tuple[Tensor, Tensor]:
+        (img_mod1, img_mod2), (txt_mod1, txt_mod2) = distill_vec
+
+        # prepare image for attention
+        img_modulated = self.img_norm1(img)
+        # replaced with compiled fn
+        # img_modulated = (1 + img_mod1.scale) * img_modulated + img_mod1.shift
+        img_modulated = self.modulation_shift_scale_fn(img_modulated, img_mod1.scale, img_mod1.shift)
+        img_qkv = self.img_attn.qkv(img_modulated)
+        img_q, img_k, img_v = rearrange(img_qkv, "B L (K H D) -> K B H L D", K=3, H=self.num_heads)
+        img_q, img_k = self.img_attn.norm(img_q, img_k, img_v)
+
+        # prepare txt for attention
+        txt_modulated = self.txt_norm1(txt)
+        # replaced with compiled fn
+        # txt_modulated = (1 + txt_mod1.scale) * txt_modulated + txt_mod1.shift
+        txt_modulated = self.modulation_shift_scale_fn(txt_modulated, txt_mod1.scale, txt_mod1.shift)
+        txt_qkv = self.txt_attn.qkv(txt_modulated)
+        txt_q, txt_k, txt_v = rearrange(txt_qkv, "B L (K H D) -> K B H L D", K=3, H=self.num_heads)
+        txt_q, txt_k = self.txt_attn.norm(txt_q, txt_k, txt_v)
+
+        # run actual attention
+        q = torch.cat((txt_q, img_q), dim=2)
+        k = torch.cat((txt_k, img_k), dim=2)
+        v = torch.cat((txt_v, img_v), dim=2)
+
+        attn = attention(q, k, v, pe=pe, mask=mask)
+        txt_attn, img_attn = attn[:, : txt.shape[1]], attn[:, txt.shape[1] :]
+
+        # calculate the img bloks
+        # replaced with compiled fn
+        # img = img + img_mod1.gate * self.img_attn.proj(img_attn)
+        # img = img + img_mod2.gate * self.img_mlp((1 + img_mod2.scale) * self.img_norm2(img) + img_mod2.shift)
+        img = self.modulation_gate_fn(img, img_mod1.gate, self.img_attn.proj(img_attn))
+        img = self.modulation_gate_fn(
+            img,
+            img_mod2.gate,
+            self.img_mlp(self.modulation_shift_scale_fn(self.img_norm2(img), img_mod2.scale, img_mod2.shift)),
+        )
+
+        # calculate the txt bloks
+        # replaced with compiled fn
+        # txt = txt + txt_mod1.gate * self.txt_attn.proj(txt_attn)
+        # txt = txt + txt_mod2.gate * self.txt_mlp((1 + txt_mod2.scale) * self.txt_norm2(txt) + txt_mod2.shift)
+        txt = self.modulation_gate_fn(txt, txt_mod1.gate, self.txt_attn.proj(txt_attn))
+        txt = self.modulation_gate_fn(
+            txt,
+            txt_mod2.gate,
+            self.txt_mlp(self.modulation_shift_scale_fn(self.txt_norm2(txt), txt_mod2.scale, txt_mod2.shift)),
+        )
+
+        return img, txt
+
+    def forward(
+        self,
+        img: Tensor,
+        txt: Tensor,
+        pe: Tensor,
+        distill_vec: list[ModulationOut],
+        mask: Tensor,
+    ) -> tuple[Tensor, Tensor]:
+        if self.training and self.gradient_checkpointing:
+            return ckpt.checkpoint(self._forward, img, txt, pe, distill_vec, mask, use_reentrant=False)
+        else:
+            return self._forward(img, txt, pe, distill_vec, mask)
+
+
+class SingleStreamBlock(nn.Module):
+    """
+    A DiT block with parallel linear layers as described in
+    https://arxiv.org/abs/2302.05442 and adapted modulation interface.
+    """
+
+    def __init__(
+        self,
+        hidden_size: int,
+        num_heads: int,
+        mlp_ratio: float = 4.0,
+        qk_scale: float | None = None,
+    ):
+        super().__init__()
+        self.hidden_dim = hidden_size
+        self.num_heads = num_heads
+        head_dim = hidden_size // num_heads
+        self.scale = qk_scale or head_dim**-0.5
+
+        self.mlp_hidden_dim = int(hidden_size * mlp_ratio)
+        # qkv and mlp_in
+        self.linear1 = nn.Linear(hidden_size, hidden_size * 3 + self.mlp_hidden_dim)
+        # proj and mlp_out
+        self.linear2 = nn.Linear(hidden_size + self.mlp_hidden_dim, hidden_size)
+
+        self.norm = QKNorm(head_dim)
+
+        self.hidden_size = hidden_size
+        self.pre_norm = nn.LayerNorm(hidden_size, elementwise_affine=False, eps=1e-6)
+
+        self.mlp_act = nn.GELU(approximate="tanh")
+
+        self.gradient_checkpointing = False
+
+    @property
+    def device(self):
+        # Get the device of the module (assumes all parameters are on the same device)
+        return next(self.parameters()).device
+
+    def modulation_shift_scale_fn(self, x, scale, shift):
+        return _modulation_shift_scale_fn(x, scale, shift)
+
+    def modulation_gate_fn(self, x, gate, gate_params):
+        return _modulation_gate_fn(x, gate, gate_params)
+
+    def enable_gradient_checkpointing(self):
+        self.gradient_checkpointing = True
+
+    def disable_gradient_checkpointing(self):
+        self.gradient_checkpointing = False
+
+    def _forward(self, x: Tensor, pe: Tensor, distill_vec: list[ModulationOut], mask: Tensor) -> Tensor:
+        mod = distill_vec
+        # replaced with compiled fn
+        # x_mod = (1 + mod.scale) * self.pre_norm(x) + mod.shift
+        x_mod = self.modulation_shift_scale_fn(self.pre_norm(x), mod.scale, mod.shift)
+        qkv, mlp = torch.split(self.linear1(x_mod), [3 * self.hidden_size, self.mlp_hidden_dim], dim=-1)
+
+        q, k, v = rearrange(qkv, "B L (K H D) -> K B H L D", K=3, H=self.num_heads)
+        q, k = self.norm(q, k, v)
+
+        # compute attention
+        attn = attention(q, k, v, pe=pe, mask=mask)
+        # compute activation in mlp stream, cat again and run second linear layer
+        output = self.linear2(torch.cat((attn, self.mlp_act(mlp)), 2))
+        # replaced with compiled fn
+        # return x + mod.gate * output
+        return self.modulation_gate_fn(x, mod.gate, output)
+
+    def forward(self, x: Tensor, pe: Tensor, distill_vec: list[ModulationOut], mask: Tensor) -> Tensor:
+        if self.training and self.gradient_checkpointing:
+            return ckpt.checkpoint(self._forward, x, pe, distill_vec, mask, use_reentrant=False)
+        else:
+            return self._forward(x, pe, distill_vec, mask)
+
+
+class LastLayer(nn.Module):
+    def __init__(
+        self,
+        hidden_size: int,
+        patch_size: int,
+        out_channels: int,
+    ):
+        super().__init__()
+        self.norm_final = nn.LayerNorm(hidden_size, elementwise_affine=False, eps=1e-6)
+        self.linear = nn.Linear(hidden_size, patch_size * patch_size * out_channels, bias=True)
+
+    @property
+    def device(self):
+        # Get the device of the module (assumes all parameters are on the same device)
+        return next(self.parameters()).device
+
+    def modulation_shift_scale_fn(self, x, scale, shift):
+        return _modulation_shift_scale_fn(x, scale, shift)
+
+    def forward(self, x: Tensor, distill_vec: list[Tensor]) -> Tensor:
+        shift, scale = distill_vec
+        shift = shift.squeeze(1)
+        scale = scale.squeeze(1)
+        # replaced with compiled fn
+        # x = (1 + scale[:, None, :]) * self.norm_final(x) + shift[:, None, :]
+        x = self.modulation_shift_scale_fn(self.norm_final(x), scale[:, None, :], shift[:, None, :])
+        x = self.linear(x)
+        return x
+
+
+@dataclass
+class ChromaParams:
+    in_channels: int
+    context_in_dim: int
+    hidden_size: int
+    mlp_ratio: float
+    num_heads: int
+    depth: int
+    depth_single_blocks: int
+    axes_dim: list[int]
+    theta: int
+    qkv_bias: bool
+    guidance_embed: bool
+    approximator_in_dim: int
+    approximator_depth: int
+    approximator_hidden_size: int
+    _use_compiled: bool
+
+
+chroma_params = ChromaParams(
+    in_channels=64,
+    context_in_dim=4096,
+    hidden_size=3072,
+    mlp_ratio=4.0,
+    num_heads=24,
+    depth=19,
+    depth_single_blocks=38,
+    axes_dim=[16, 56, 56],
+    theta=10_000,
+    qkv_bias=True,
+    guidance_embed=True,
+    approximator_in_dim=64,
+    approximator_depth=5,
+    approximator_hidden_size=5120,
+    _use_compiled=False,
+)
+
+
+def modify_mask_to_attend_padding(mask, max_seq_length, num_extra_padding=8):
+    """
+    Modifies attention mask to allow attention to a few extra padding tokens.
+
+    Args:
+        mask: Original attention mask (1 for tokens to attend to, 0 for masked tokens)
+        max_seq_length: Maximum sequence length of the model
+        num_extra_padding: Number of padding tokens to unmask
+
+    Returns:
+        Modified mask
+    """
+    # Get the actual sequence length from the mask
+    seq_length = mask.sum(dim=-1)
+    batch_size = mask.shape[0]
+
+    modified_mask = mask.clone()
+
+    for i in range(batch_size):
+        current_seq_len = int(seq_length[i].item())
+
+        # Only add extra padding tokens if there's room
+        if current_seq_len < max_seq_length:
+            # Calculate how many padding tokens we can unmask
+            available_padding = max_seq_length - current_seq_len
+            tokens_to_unmask = min(num_extra_padding, available_padding)
+
+            # Unmask the specified number of padding tokens right after the sequence
+            modified_mask[i, current_seq_len : current_seq_len + tokens_to_unmask] = 1
+
+    return modified_mask
+
+
+class Chroma(nn.Module):
+    """
+    Transformer model for flow matching on sequences.
+    """
+
+    def __init__(self, params: ChromaParams):
+        super().__init__()
+        self.params = params
+        self.in_channels = params.in_channels
+        self.out_channels = self.in_channels
+        if params.hidden_size % params.num_heads != 0:
+            raise ValueError(f"Hidden size {params.hidden_size} must be divisible by num_heads {params.num_heads}")
+        pe_dim = params.hidden_size // params.num_heads
+        if sum(params.axes_dim) != pe_dim:
+            raise ValueError(f"Got {params.axes_dim} but expected positional dim {pe_dim}")
+        self.hidden_size = params.hidden_size
+        self.num_heads = params.num_heads
+        self.pe_embedder = EmbedND(dim=pe_dim, theta=params.theta, axes_dim=params.axes_dim)
+        self.img_in = nn.Linear(self.in_channels, self.hidden_size, bias=True)
+
+        # TODO: need proper mapping for this approximator output!
+        # currently the mapping is hardcoded in distribute_modulations function
+        self.distilled_guidance_layer = Approximator(
+            params.approximator_in_dim,
+            self.hidden_size,
+            params.approximator_hidden_size,
+            params.approximator_depth,
+        )
+        self.txt_in = nn.Linear(params.context_in_dim, self.hidden_size)
+
+        self.double_blocks = nn.ModuleList(
+            [
+                DoubleStreamBlock(
+                    self.hidden_size,
+                    self.num_heads,
+                    mlp_ratio=params.mlp_ratio,
+                    qkv_bias=params.qkv_bias,
+                )
+                for _ in range(params.depth)
+            ]
+        )
+
+        self.single_blocks = nn.ModuleList(
+            [
+                SingleStreamBlock(
+                    self.hidden_size,
+                    self.num_heads,
+                    mlp_ratio=params.mlp_ratio,
+                )
+                for _ in range(params.depth_single_blocks)
+            ]
+        )
+
+        self.final_layer = LastLayer(
+            self.hidden_size,
+            1,
+            self.out_channels,
+        )
+
+        # TODO: move this hardcoded value to config
+        # single layer has 3 modulation vectors
+        # double layer has 6 modulation vectors for each expert
+        # final layer has 2 modulation vectors
+        self.mod_index_length = 3 * params.depth_single_blocks + 2 * 6 * params.depth + 2
+        self.depth_single_blocks = params.depth_single_blocks
+        self.depth_double_blocks = params.depth
+        # self.mod_index = torch.tensor(list(range(self.mod_index_length)), device=0)
+        self.register_buffer(
+            "mod_index",
+            torch.tensor(list(range(self.mod_index_length)), device="cpu"),
+            persistent=False,
+        )
+        self.approximator_in_dim = params.approximator_in_dim
+
+        self.blocks_to_swap = None
+        self.offloader_double = None
+        self.offloader_single = None
+        self.num_double_blocks = len(self.double_blocks)
+        self.num_single_blocks = len(self.single_blocks)
+
+    @property
+    def device(self):
+        # Get the device of the module (assumes all parameters are on the same device)
+        return next(self.parameters()).device
+
+    def enable_gradient_checkpointing(self):
+        self.distilled_guidance_layer.enable_gradient_checkpointing()
+        for block in self.double_blocks + self.single_blocks:
+            block.enable_gradient_checkpointing()
+
+    def disable_gradient_checkpointing(self):
+        self.distilled_guidance_layer.disable_gradient_checkpointing()
+        for block in self.double_blocks + self.single_blocks:
+            block.disable_gradient_checkpointing()
+
+    def enable_block_swap(self, num_blocks: int, device: torch.device):
+        self.blocks_to_swap = num_blocks
+        double_blocks_to_swap = num_blocks // 2
+        single_blocks_to_swap = (num_blocks - double_blocks_to_swap) * 2
+
+        assert double_blocks_to_swap <= self.num_double_blocks - 2 and single_blocks_to_swap <= self.num_single_blocks - 2, (
+            f"Cannot swap more than {self.num_double_blocks - 2} double blocks and {self.num_single_blocks - 2} single blocks. "
+            f"Requested {double_blocks_to_swap} double blocks and {single_blocks_to_swap} single blocks."
+        )
+
+        self.offloader_double = custom_offloading_utils.ModelOffloader(
+            self.double_blocks, double_blocks_to_swap, device
+        )
+        self.offloader_single = custom_offloading_utils.ModelOffloader(
+            self.single_blocks, single_blocks_to_swap, device
+        )
+        print(
+            f"Chroma: Block swap enabled. Swapping {num_blocks} blocks, double blocks: {double_blocks_to_swap}, single blocks: {single_blocks_to_swap}."
+        )
+
+    def move_to_device_except_swap_blocks(self, device: torch.device):
+        # assume model is on cpu. do not move blocks to device to reduce temporary memory usage
+        if self.blocks_to_swap:
+            save_double_blocks = self.double_blocks
+            save_single_blocks = self.single_blocks
+            self.double_blocks = None
+            self.single_blocks = None
+
+        self.to(device)
+
+        if self.blocks_to_swap:
+            self.double_blocks = save_double_blocks
+            self.single_blocks = save_single_blocks
+
+    def prepare_block_swap_before_forward(self):
+        if self.blocks_to_swap is None or self.blocks_to_swap == 0:
+            return
+        self.offloader_double.prepare_block_devices_before_forward(self.double_blocks)
+        self.offloader_single.prepare_block_devices_before_forward(self.single_blocks)
+
+    def forward(
+        self,
+        img: Tensor,
+        img_ids: Tensor,
+        txt: Tensor,
+        txt_ids: Tensor,
+        txt_mask: Tensor,
+        timesteps: Tensor,
+        guidance: Tensor,
+        attn_padding: int = 1,
+    ) -> Tensor:
+        if img.ndim != 3 or txt.ndim != 3:
+            raise ValueError("Input img and txt tensors must have 3 dimensions.")
+
+        # running on sequences img
+        img = self.img_in(img)
+        txt = self.txt_in(txt)
+
+        # TODO:
+        # need to fix grad accumulation issue here for now it's in no grad mode
+        # besides, i don't want to wash out the PFP that's trained on this model weights anyway
+        # the fan out operation here is deleting the backward graph
+        # alternatively doing forward pass for every block manually is doable but slow
+        # custom backward probably be better
+        with torch.no_grad():
+            distill_timestep = timestep_embedding(timesteps, self.approximator_in_dim // 4)
+            # TODO: need to add toggle to omit this from schnell but that's not a priority
+            distil_guidance = timestep_embedding(guidance, self.approximator_in_dim // 4)
+            # get all modulation index
+            modulation_index = timestep_embedding(self.mod_index, self.approximator_in_dim // 2)
+            # we need to broadcast the modulation index here so each batch has all of the index
+            modulation_index = modulation_index.unsqueeze(0).repeat(img.shape[0], 1, 1)
+            # and we need to broadcast timestep and guidance along too
+            timestep_guidance = (
+                torch.cat([distill_timestep, distil_guidance], dim=1).unsqueeze(1).repeat(1, self.mod_index_length, 1)
+            )
+            # then and only then we could concatenate it together
+            input_vec = torch.cat([timestep_guidance, modulation_index], dim=-1)
+            mod_vectors = self.distilled_guidance_layer(input_vec.requires_grad_(True))
+        mod_vectors_dict = distribute_modulations(mod_vectors, self.depth_single_blocks, self.depth_double_blocks)
+
+        ids = torch.cat((txt_ids, img_ids), dim=1)
+        pe = self.pe_embedder(ids)
+
+        # compute mask
+        # assume max seq length from the batched input
+
+        max_len = txt.shape[1]
+
+        # mask
+        with torch.no_grad():
+            txt_mask_w_padding = modify_mask_to_attend_padding(txt_mask, max_len, attn_padding)
+            txt_img_mask = torch.cat(
+                [
+                    txt_mask_w_padding,
+                    torch.ones([img.shape[0], img.shape[1]], device=txt_mask.device),
+                ],
+                dim=1,
+            )
+            txt_img_mask = txt_img_mask.float().T @ txt_img_mask.float()
+            txt_img_mask = txt_img_mask[None, None, ...].repeat(txt.shape[0], self.num_heads, 1, 1).int().bool()
+            # txt_mask_w_padding[txt_mask_w_padding==False] = True
+
+        if not self.blocks_to_swap:
+            for i, block in enumerate(self.double_blocks):
+                # the guidance replaced by FFN output
+                img_mod = mod_vectors_dict[f"double_blocks.{i}.img_mod.lin"]
+                txt_mod = mod_vectors_dict[f"double_blocks.{i}.txt_mod.lin"]
+                double_mod = [img_mod, txt_mod]
+
+                img, txt = block(img=img, txt=txt, pe=pe, distill_vec=double_mod, mask=txt_img_mask)
+        else:
+            for i, block in enumerate(self.double_blocks):
+                self.offloader_double.wait_for_block(i)
+
+                # the guidance replaced by FFN output
+                img_mod = mod_vectors_dict[f"double_blocks.{i}.img_mod.lin"]
+                txt_mod = mod_vectors_dict[f"double_blocks.{i}.txt_mod.lin"]
+                double_mod = [img_mod, txt_mod]
+
+                img, txt = block(img=img, txt=txt, pe=pe, distill_vec=double_mod, mask=txt_img_mask)
+
+                self.offloader_double.submit_move_blocks(self.double_blocks, i)
+
+        img = torch.cat((txt, img), 1)
+        if not self.blocks_to_swap:
+            for i, block in enumerate(self.single_blocks):
+                single_mod = mod_vectors_dict[f"single_blocks.{i}.modulation.lin"]
+                img = block(img, pe=pe, distill_vec=single_mod, mask=txt_img_mask)
+        else:
+            for i, block in enumerate(self.single_blocks):
+                self.offloader_single.wait_for_block(i)
+
+                single_mod = mod_vectors_dict[f"single_blocks.{i}.modulation.lin"]
+                img = block(img, pe=pe, distill_vec=single_mod, mask=txt_img_mask)
+
+                self.offloader_single.submit_move_blocks(self.single_blocks, i)
+        img = img[:, txt.shape[1] :, ...]
+        final_mod = mod_vectors_dict["final_layer.adaLN_modulation.1"]
+        img = self.final_layer(img, distill_vec=final_mod)  # (N, T, patch_size ** 2 * out_channels)
+        return img