[bugfix]: bound WanCausalModel num_frames_per_block override to <= 3

The constructor override only checked >= 1, bypassing the <= 3 limit the
config path enforces in CausalWanTransformer3DModel. Apply the same upper
bound with a clear error message.

fastvideo/train/models/wan/wan_causal.py

@@ -64,9 +64,14 @@ class WanCausalModel(WanModel, CausalModelBase):
         self._streaming_caches: (dict[tuple[int, str], _StreamingCaches]) = {}
 
         if num_frames_per_block is not None:
-            if int(num_frames_per_block) < 1:
-                raise ValueError("num_frames_per_block must be >= 1")
-            self.transformer.num_frame_per_block = int(num_frames_per_block)
+            num_frames_per_block = int(num_frames_per_block)
+            if not 1 <= num_frames_per_block <= 3:
+                # Same bound as CausalWanTransformer3DModel's config path
+                # (assert num_frame_per_block <= 3); this override must not
+                # bypass it.
+                raise ValueError("num_frames_per_block must be between 1 and 3, "
+                                 f"got {num_frames_per_block}")
+            self.transformer.num_frame_per_block = num_frames_per_block
 
     # --- CausalModelBase override: clear_caches ---
     def clear_caches(