feat: reduce memory usage and add memory efficient option for model saving

flux_train.py

@@ -759,6 +759,12 @@ def setup_parser() -> argparse.ArgumentParser:
     add_custom_train_arguments(parser)  # TODO remove this from here
     flux_train_utils.add_flux_train_arguments(parser)
 
+    parser.add_argument(
+        "--mem_eff_save",
+        action="store_true",
+        help="[EXPERIMENTAL] use memory efficient custom model saving method / メモリ効率の良い独自のモデル保存方法を使う",
+    )
+
     parser.add_argument(
         "--fused_optimizer_groups",
         type=int,