Fail on num_inference_steps set for distilled models

Author: yzhautouskay

src/diffusers/modular_pipelines/cosmos/before_denoise.py

@@ -945,7 +945,7 @@ def expected_components(self) -> list[ComponentSpec]:
     @property
     def inputs(self) -> list[InputParam]:
         return [
-            InputParam.template("num_inference_steps", required=False),
+            InputParam.template("num_inference_steps", required=False, default=None),
             InputParam(
                 name="guidance_scale",
                 type_hint=float,

src/diffusers/modular_pipelines/cosmos/modular_pipeline.py

@@ -105,6 +105,12 @@ def resolve_inference_schedule(
         """Resolve step count and guidance for base vs distilled checkpoints."""
         if self.is_distilled_checkpoint:
             distilled_steps = len(self.distilled_sigmas)
+            if num_inference_steps is not None and num_inference_steps != distilled_steps:
+                raise ValueError(
+                    "This is a distilled checkpoint; the step count is fixed by the scheduler's "
+                    f"`fixed_step_sampler_config.t_list` ({distilled_steps} steps). "
+                    f"`num_inference_steps` must be {distilled_steps} or left unset (got {num_inference_steps})."
+                )
             if guidance_scale is not None and guidance_scale != 1.0:
                 raise ValueError(
                     "This is a distilled checkpoint; classifier-free guidance is baked into the weights. "