[Feature] Add free threading support

[kevmo314]

Description

This change removes a race condition in value caching that causes segfaults in python under free threading.

Additionally, it adds a declaration that the library does not use the GIL.

Motivation and Context

tensordict segfaults with this script on py3.14t with PYTHON_GIL=0.

import sys
import gc
import threading

import torch
from tensordict import TensorDict


def gc_stress():
    for _ in range(100):
        td = TensorDict(
            {"a": torch.randn(5, 5), "b": {"c": torch.randn(5, 3)}}, batch_size=[5]
        )
        td = None
        gc.collect()


def main():
    print(f"Python version: {sys.version}")
    if hasattr(sys, "_is_gil_enabled"):
        print(f"GIL enabled: {sys._is_gil_enabled()}")
    else:
        print("GIL status: N/A (not a free-threading build)")

    threads = [threading.Thread(target=gc_stress) for _ in range(8)]
    for t in threads:
        t.start()
    for t in threads:
        t.join()
    print("Done - no crash!")


if __name__ == "__main__":
    main()

I can put this in a test if you'd like. It's more of a regression test than a unit test.

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

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)

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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.

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[kevmo314]

I've signed the CLA.

[vmoens]

LGTM in principle but let's wait till pytorch/pytorch#169033 is resolved