refactor(converters): gather converters and refactor the logic (#1833)

* refactor(converters): move batch transition functions to converters module

- Moved `_default_batch_to_transition` and `_default_transition_to_batch` functions from `pipeline.py` to `converters.py` for better organization and separation of concerns.
- Updated references in `RobotProcessor` to use the new location of these functions.
- Added tests to ensure correct functionality of the transition functions, including handling of index and task_index fields.
- Removed redundant tests from `pipeline.py` to streamline the test suite.

* refactor(processor): reorganize EnvTransition and TransitionKey definitions

- Moved `EnvTransition` and `TransitionKey` classes from `pipeline.py` to a new `core.py` module for better structure and maintainability.
- Updated import statements across relevant modules to reflect the new location of these definitions, ensuring consistent access throughout the codebase.

* refactor(converters): rename and update dataset frame conversion functions

- Replaced `to_dataset_frame` with `transition_to_dataset_frame` for clarity and consistency in naming.
- Updated references in `record.py`, `pipeline.py`, and tests to use the new function name.
- Introduced `merge_transitions` to streamline the merging of transitions, enhancing readability and maintainability.
- Adjusted related tests to ensure correct functionality with the new naming conventions.

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* fix(processor): solve conflict artefacts

* refactor(converters): remove unused identity function and update type hints for merge_transitions

* refactor(processor): remove unused identity import and clean up gym_manipulator.py

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Steven Palma <steven.palma@huggingface.co>

src/lerobot/processor/__init__.py

@@ -15,6 +15,14 @@
 # limitations under the License.
 
 from .batch_processor import ToBatchProcessor
+from .converters import (
+    batch_to_transition,
+    create_transition,
+    merge_transitions,
+    transition_to_batch,
+    transition_to_dataset_frame,
+)
+from .core import EnvTransition, TransitionKey
 from .delta_action_processor import MapDeltaActionToRobotAction, MapTensorToDeltaActionDict
 from .device_processor import DeviceProcessor
 from .gym_action_processor import Numpy2TorchActionProcessor, Torch2NumpyActionProcessor
@@ -33,7 +41,6 @@ from .observation_processor import VanillaObservationProcessor
 from .pipeline import (
     ActionProcessor,
     DoneProcessor,
-    EnvTransition,
     IdentityProcessor,
     InfoProcessor,
     ObservationProcessor,
@@ -42,7 +49,6 @@ from .pipeline import (
     ProcessorStepRegistry,
     RewardProcessor,
     RobotProcessor,
-    TransitionKey,
     TruncatedProcessor,
 )
 from .rename_processor import RenameProcessor
@@ -52,22 +58,24 @@ __all__ = [
     "ActionProcessor",
     "AddTeleopActionAsComplimentaryData",
     "AddTeleopEventsAsInfo",
+    "batch_to_transition",
+    "create_transition",
     "DeviceProcessor",
     "DoneProcessor",
-    "MapDeltaActionToRobotAction",
-    "MapTensorToDeltaActionDict",
     "EnvTransition",
     "GripperPenaltyProcessor",
+    "hotswap_stats",
     "IdentityProcessor",
     "ImageCropResizeProcessor",
     "InfoProcessor",
     "InterventionActionProcessor",
     "JointVelocityProcessor",
     "MapDeltaActionToRobotAction",
+    "MapTensorToDeltaActionDict",
+    "merge_transitions",
     "MotorCurrentProcessor",
     "NormalizerProcessor",
-    "UnnormalizerProcessor",
-    "hotswap_stats",
+    "Numpy2TorchActionProcessor",
     "ObservationProcessor",
     "ProcessorKwargs",
     "ProcessorStep",
@@ -76,12 +84,14 @@ __all__ = [
     "RewardClassifierProcessor",
     "RewardProcessor",
     "RobotProcessor",
+    "TimeLimitProcessor",
     "ToBatchProcessor",
     "TokenizerProcessor",
-    "TimeLimitProcessor",
-    "Numpy2TorchActionProcessor",
     "Torch2NumpyActionProcessor",
+    "transition_to_batch",
+    "transition_to_dataset_frame",
     "TransitionKey",
     "TruncatedProcessor",
+    "UnnormalizerProcessor",
     "VanillaObservationProcessor",
 ]