refactor(pipeline): enforce ProcessorStep inheritance for pipeline steps (#1862)

- Updated the DataProcessorPipeline to require that all steps inherit from ProcessorStep, enhancing type safety and clarity.
- Adjusted tests to utilize a MockTokenizerProcessorStep that adheres to the ProcessorStep interface, ensuring consistent behavior across tests.
- Refactored various mock step classes in tests to inherit from ProcessorStep for improved consistency and maintainability.

src/lerobot/processor/pipeline.py

@@ -731,11 +731,8 @@ class DataProcessorPipeline(ModelHubMixin, Generic[TOutput]):
 
     def __post_init__(self):
         for i, step in enumerate(self.steps):
-            if not callable(step):
-                # TODO(steven): This should instead check isinstance(step, ProcessorStep), test need to be updated
-                raise TypeError(
-                    f"Step {i} ({type(step).__name__}) must define __call__(transition) -> EnvTransition"
-                )
+            if not isinstance(step, ProcessorStep):
+                raise TypeError(f"Step {i} ({type(step).__name__}) must inherit from ProcessorStep")
 
     def transform_features(self, initial_features: dict[str, PolicyFeature]) -> dict[str, PolicyFeature]:
         """