[Perf][Diffusion] Add LTX2 channels-last VAE path

Signed-off-by: Hongsheng Liu <liuhongsheng4@huawei.com>

vllm_omni/diffusion/data.py

@@ -711,6 +711,8 @@ class OmniDiffusionConfig:
     # VAE memory optimization parameters
     vae_use_slicing: bool = False
     vae_use_tiling: bool = False
+    # Opt in because CL3D performance varies materially by model shape and GPU.
+    vae_use_channels_last_3d: bool = False
 
     # STA (Sliding Tile Attention) parameters
     mask_strategy_file_path: str | None = None
@@ -930,6 +932,8 @@ class OmniDiffusionConfig:
             )
         if not isinstance(self.diffusion_compile_dynamic, bool):
             raise TypeError(f"diffusion_compile_dynamic must be a bool, got {type(self.diffusion_compile_dynamic)!r}")
+        if not isinstance(self.vae_use_channels_last_3d, bool):
+            raise TypeError(f"vae_use_channels_last_3d must be a bool, got {type(self.vae_use_channels_last_3d)!r}")
         self.diffusion_kv_mode = parse_diffusion_kv_cache_mode(self.diffusion_kv_mode)
 
         if self.omni_kv_config is None: