[core][GPU Objects] Add related tests for tensordict

[Qiaolin-Yu]

Why are these changes needed?

The current code already supports extracting tensors from tensordict, and using external transport to transfer the tensors. This PR adds some tests to prove this (e.g., checking the extracted tensor stored in the gpu object manager, and testing nested tensordict).

Related issue number

close #52340

Checks

• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[Copilot]

Pull Request Overview

This PR adds tests to validate TensorDict support in GPU object transfers using the Gloo backend.

• Import TensorDict and extend GPUTestActor.double to handle TensorDict inputs.
• Enhance test_multiple_tensors to include TensorDict items.
• Introduce three new tests:
  • test_tensordict_transfer
  • test_nested_tensordict
  • test_tensor_extracted_from_tensordict_in_gpu_object_store

Comments suppressed due to low confidence (1)

python/ray/tests/test_gpu_objects_gloo.py:232

• The code calls sys.exit in the __main__ guard but sys is not imported at the top of the file. Add import sys to avoid a NameError.

if __name__ == "__main__":

[Qiaolin-Yu]

@jjyao , would you mind merging this?