[Feature] Add TensorClassModuleBase for type-safe TensorClass modules

[az0uz]

Description

This PR introduces TensorClassModuleBase, a new base class that provides a type-safe way to define PyTorch modules that operate on TensorClass instances. This implementation offers compile-time type checking and seamless integration with TensorDict-based workflows.

Key Features:

• Type-safe module definition: Generic base class TensorClassModuleBase[InputClass, OutputClass] with automatic type extraction
• TensorDict integration: TensorClassModuleWrapper enables conversion to TensorDictModule via as_td_module() method
• Nested TensorClass support: Automatically handles nested TensorClass structures and extracts all keys
• ONNX export compatibility: Full support for ONNX export with proper input/output handling

Implementation Details:

• New module: tensordict/nn/tensorclass_module.py with TensorClassModuleBase and TensorClassModuleWrapper
• Automatic key extraction from TensorClass type annotations using _tensor_class_keys()
• Generic type parameter extraction via __init_subclass__ for clean API
• Comprehensive test suite in test/test_tensorclass_module.py covering forward pass, TensorDict conversion, ONNX export, and edge cases

Additional Changes:

• Exported new classes in tensordict/__init__.py and tensordict/nn/__init__.py

Motivation and Context

• I have raised an issue to propose this change (required for new features and bug fixes)

implements #1355

This change addresses the need for type-safe module definitions when working with TensorClass instances. Previously, developers had to work directly with TensorDict or manually handle type conversions, which was error-prone and lacked compile-time type checking.

With TensorClassModuleBase, developers can:

1. Define modules with explicit input/output TensorClass types
2. Get IDE autocomplete and type checking for TensorClass fields
3. Seamlessly integrate with existing TensorDict-based workflows
4. Export to ONNX while maintaining type safety

Types of changes

What types of changes does your code introduce? Remove all that do not apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds core functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (update in the documentation)
• Example (update in the folder of examples)

Checklist

Go over all the following points, and put an x in all the boxes that apply.
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• I have read the CONTRIBUTION guide (required)
• My change requires a change to the documentation.
• I have updated the tests accordingly (required for a bug fix or a new feature).
• I have updated the documentation accordingly (via comprehensive docstrings and examples).

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[vmoens]

Thanks for this!
Overall great feature!
I left some comments here and there but they're pretty minor.
Can you sign the CLA? It's required to run the tests and merge

[vmoens]

new classes need to be added to the doc

[az0uz]

Do I need to write anything else than adding them to docs/source/reference/nn.rst like above?

I've added a section with description and examples. let me know if that works for you.

[vmoens]

That's really cool thx

[vmoens]

Thanks for this!
Overall great feature!
I left some comments here and there but they're pretty minor.
Can you sign the CLA? It's required to run the tests and merge

[az0uz]

The person responsible for the existing cla in my company has asked cla@ to add me, but haven't got any answer yet, I'll ping them again. Do I need to also sign it personally?

[vmoens]

The person responsible for the existing cla in my company has asked cla@ to add me, but haven't got any answer yet, I'll ping them again. Do I need to also sign it personally?

No the company one should be fine. If we need to ping anyone on the Meta side LMK!

[az0uz]

If we need to ping anyone on the Meta side LMK!

Yes that would be great, no answer for a week, we've tried to ping cla@meta.com again yesterday but still no news.

[vmoens]

GHA seems down -- we'll need to wait till tests run for this

[vmoens]

looking good!

[vmoens]

That's really cool thx