[BugFix] Convert Mapping values for TensorDict fields in tensor-only tensorclasses

[peterdsharpe]

Description

This PR converts arbitrary collections.abc.Mapping values assigned to
TensorDict-annotated fields on tensor_only=True tensorclasses.

The annotation-aware conversion applies consistently during construction,
attribute assignment, .set(), and from_dataclass. It also:

• recursively normalizes nested mappings;
• supports generic and optional/union TensorDict annotations;
• preserves existing TensorDict instances, including identity and lock state;
• leaves mappings annotated as Any or NonTensorData unchanged;
• preserves the direct constructor fast path when no relevant fields exist.

Motivation and Context

Closes #1752.

Non-dict Mapping implementations were previously wrapped as NonTensorData
even when the destination field was annotated as TensorDict. The fix is
scoped using resolved field annotations so that it does not globally reinterpret
mappings intended to remain non-tensor data.

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

Test plan

Regression tests cover:

• dict and UserDict inputs;
• nested mappings;
• construction, attribute assignment, and .set();
• generic and optional TensorDict annotations;
• class and instance forms of from_dataclass;
• preservation of existing and nested locked TensorDict instances;
• Any and NonTensorData mapping fields remaining non-tensor.

Local validation:

• pytest test/tensorclass/test_tensorclass.py -k "TestTensorOnly":
  16 passed
• pytest test/tensorclass/test_tensorclass.py -k "not multiprocessing":
  160 passed, 1 skipped, 1 deselected
• pytest test/tensorclass/test_compatibility.py test/tensorclass/test_typedtensordict.py:
  174 passed, 14 skipped
• Ruff and git diff --check pass

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist

• 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.

[meta-cla]

Hi @peterdsharpe!

Thank you for your pull request and welcome to our community.

Action Required

In order to merge any pull request (code, docs, etc.), we require contributors to sign our Contributor License Agreement, and we don't seem to have one on file for you.

Process

In order for us to review and merge your suggested changes, please sign at https://code.facebook.com/cla. If you are contributing on behalf of someone else (eg your employer), the individual CLA may not be sufficient and your employer may need to sign the corporate CLA.

Once the CLA is signed, our tooling will perform checks and validations. Afterwards, the pull request will be tagged with CLA signed. The tagging process may take up to 1 hour after signing. Please give it that time before contacting us about it.

If you have received this in error or have any questions, please contact us at cla@meta.com. Thanks!

[meta-cla]

Thank you for signing our Contributor License Agreement. We can now accept your code for this (and any) Meta Open Source project. Thanks!

[vmoens]

Made a small edit in 849271b, otherwise lgtm!