chore(docs): Processor doc (#1685)

* chore(docs): initialize doc

* Added script for the second part of the processor doc

* precommit style nit

* improved part 2 of processor guide

* Add comprehensive documentation for processors in robotics

- Introduced a detailed guide on processors, covering their role in transforming raw robot data into model-ready inputs and vice versa.
- Explained core concepts such as EnvTransition, ProcessorStep, and RobotProcessor, along with their functionalities.
- Included examples of common processor steps like normalization, device management, batch processing, and text tokenization.
- Provided insights on building complete pipelines, integrating processors into training loops, and saving/loading configurations.
- Emphasized best practices and advanced features for effective usage of processors in robotics applications.

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

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* feat(docs): Enhance introduction to processors with additional converter functions

- Updated the introduction to processors documentation to include default batch-to-transition and transition-to-batch converters.
- Added detailed descriptions and examples for new specialized converter functions: `to_transition_teleop_action`, `to_transition_robot_observation`, `to_output_robot_action`, and `to_dataset_frame`.
- Improved clarity on how these converters facilitate integration with existing robotics applications.

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

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* Improved doc implement_your_own_pipeline
- Use normalization processor as default example
- Add section on transform features
- Add section on overrides.

* Add phone docs and use pipeline for robots/teleop docs

* Fix typo in documentation for adapters in robots/teleop section

* Enhance documentation for processors with detailed explanations and examples

- Updated the introduction to processors, clarifying the role of `EnvTransition` and `ProcessorStep`.
- Introduced `DataProcessorPipeline` as a generic orchestrator for chaining processor steps.
- Added comprehensive descriptions of new converter functions and their applications.
- Improved clarity on type safety and the differences between `RobotProcessorPipeline` and `PolicyProcessorPipeline`.
- Included examples for various processing scenarios, emphasizing best practices for data handling in robotics.

* Enhance documentation for processor migration and debugging

- Added detailed sections on the migration of models to the new `PolicyProcessorPipeline` system, including breaking changes and migration scripts.
- Introduced a comprehensive guide for debugging processor pipelines, covering common issues, step-by-step inspection, and runtime monitoring techniques.
- Updated examples to reflect new usage patterns and best practices for processor implementation and error handling.
- Clarified the role of various processor steps and their configurations in the context of robotics applications.

---------

Co-authored-by: Michel Aractingi <michel.aractingi@huggingface.co>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Pepijn <pepijn@huggingface.co>

docs/source/backwardcomp.mdx

@@ -1,5 +1,61 @@
 # Backward compatibility
 
+## Policy Normalization Migration (PR #1452)
+
+**Breaking Change**: LeRobot policies no longer have built-in normalization layers embedded in their weights. Normalization is now handled by external `PolicyProcessorPipeline` components.
+
+### What changed?
+
+|                            | Before PR #1452                                  | After PR #1452                                               |
+| -------------------------- | ------------------------------------------------ | ------------------------------------------------------------ |
+| **Normalization Location** | Embedded in model weights (`normalize_inputs.*`) | External `PolicyProcessorPipeline` components                |
+| **Model State Dict**       | Contains normalization statistics                | **Clean weights only** - no normalization parameters         |
+| **Usage**                  | `policy(batch)` handles everything               | `preprocessor(batch)` → `policy(...)` → `postprocessor(...)` |
+
+### Impact on existing models
+
+- Models trained **before** PR #1452 have normalization embedded in their weights
+- These models need migration to work with the new `PolicyProcessorPipeline` system
+- The migration extracts normalization statistics and creates separate processor pipelines
+
+### Migrating old models
+
+Use the migration script to convert models with embedded normalization:
+
+```shell
+python src/lerobot/processor/migrate_policy_normalization.py \
+    --pretrained-path lerobot/act_aloha_sim_transfer_cube_human \
+    --push-to-hub \
+    --branch migrated
+```
+
+The script:
+
+1. **Extracts** normalization statistics from model weights
+2. **Creates** external preprocessor and postprocessor pipelines
+3. **Removes** normalization layers from model weights
+4. **Saves** clean model + processor pipelines
+5. **Pushes** to Hub with automatic PR creation
+
+### Using migrated models
+
+```python
+# New usage pattern (after migration)
+from lerobot.policies.factory import make_policy, make_pre_post_processors
+
+# Load model and processors separately
+policy = make_policy(config, ds_meta=dataset.meta)
+preprocessor, postprocessor = make_pre_post_processors(
+    policy_cfg=config,
+    dataset_stats=dataset.meta.stats
+)
+
+# Process data through pipeline
+processed_batch = preprocessor(raw_batch)
+action = policy.select_action(processed_batch)
+final_action = postprocessor(action)
+```
+
 ## Hardware API redesign
 
 PR [#777](https://github.com/huggingface/lerobot/pull/777) improves the LeRobot calibration but is **not backward-compatible**. Below is a overview of what changed and how you can continue to work with datasets created before this pull request.