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.

tests/processor/test_pi0_processor.py

@@ -27,13 +27,31 @@ from lerobot.policies.pi0.processor_pi0 import Pi0NewLineProcessor, make_pi0_pre
 from lerobot.processor import (
     AddBatchDimensionProcessorStep,
     DeviceProcessorStep,
+    EnvTransition,
     NormalizerProcessorStep,
+    ProcessorStep,
     RenameProcessorStep,
     TransitionKey,
     UnnormalizerProcessorStep,
 )
 
 
+class MockTokenizerProcessorStep(ProcessorStep):
+    """Mock tokenizer processor step for testing."""
+
+    def __init__(self, *args, **kwargs):
+        # Accept any arguments to mimic the real TokenizerProcessorStep interface
+        pass
+
+    def __call__(self, transition: EnvTransition) -> EnvTransition:
+        # Pass through transition unchanged
+        return transition
+
+    def transform_features(self, features):
+        # Pass through features unchanged
+        return features
+
+
 def create_transition(observation=None, action=None, **kwargs):
     """Helper function to create a transition dictionary."""
     transition = {}
@@ -83,7 +101,7 @@ def test_make_pi0_processor_basic():
     config = create_default_config()
     stats = create_default_stats()
 
-    with patch("lerobot.policies.pi0.processor_pi0.TokenizerProcessorStep"):
+    with patch("lerobot.policies.pi0.processor_pi0.TokenizerProcessorStep", MockTokenizerProcessorStep):
         preprocessor, postprocessor = make_pi0_pre_post_processors(
             config,
             stats,
@@ -165,7 +183,7 @@ def test_pi0_processor_cuda():
     stats = create_default_stats()
 
     # Mock the tokenizer processor to act as pass-through
-    class MockTokenizerProcessorStep:
+    class MockTokenizerProcessorStep(ProcessorStep):
         def __init__(self, *args, **kwargs):
             pass
 
@@ -220,7 +238,7 @@ def test_pi0_processor_accelerate_scenario():
     stats = create_default_stats()
 
     # Mock the tokenizer processor to act as pass-through
-    class MockTokenizerProcessorStep:
+    class MockTokenizerProcessorStep(ProcessorStep):
         def __init__(self, *args, **kwargs):
             pass
 
@@ -276,7 +294,7 @@ def test_pi0_processor_multi_gpu():
     stats = create_default_stats()
 
     # Mock the tokenizer processor to act as pass-through
-    class MockTokenizerProcessorStep:
+    class MockTokenizerProcessorStep(ProcessorStep):
         def __init__(self, *args, **kwargs):
             pass
 
@@ -329,7 +347,7 @@ def test_pi0_processor_without_stats():
     config = create_default_config()
 
     # Mock the tokenizer processor
-    with patch("lerobot.policies.pi0.processor_pi0.TokenizerProcessorStep"):
+    with patch("lerobot.policies.pi0.processor_pi0.TokenizerProcessorStep", MockTokenizerProcessorStep):
         preprocessor, postprocessor = make_pi0_pre_post_processors(
             config,
             dataset_stats=None,