[docs] feat: improve docstrings in tensordict_utils.py (#1345)

[yurekami]

Summary

• Add comprehensive Google-style docstrings to 17 functions in verl/utils/tensordict_utils.py
• Improves IDE support with parameter hints, type information, and examples
• Part of the documentation standardization effort (Issue [FR][Dev] Unify (& Complete) the docstring format #1345)

Functions Documented

Non-tensor data handling:

• assign_non_tensor_data - Assign single non-tensor value to TensorDict
• unwrap_non_tensor_data - Unwrap NonTensorData to get underlying value
• get_non_tensor_data - Retrieve and unwrap non-tensor data

TensorDict concatenation/splitting:

• concat_nested_tensors - Concatenate 2D nested tensors
• concat_tensordict_with_none_bsz - Handle TensorDicts with empty batch size
• concat_tensordict - Concatenate multiple TensorDicts along dim 0
• chunk_tensordict - Split TensorDict into equal-sized chunks

Data access and manipulation:

• index_select_tensor_dict - Select rows using indices
• union_tensor_dict - Merge two TensorDicts
• make_iterator - Create mini-batch iterator for training
• assert_tensordict_eq - Assert two TensorDicts are equal
• get / get_keys - Get values with automatic unwrapping
• pop / pop_keys - Remove and return values

Padding utilities:

• pad_to_divisor - Pad batch dimension for distributed training
• unpad - Remove padding from TensorDict

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[gemini-code-assist]

Code Review

This pull request introduces comprehensive Google-style docstrings for 17 functions in verl/utils/tensordict_utils.py, significantly improving code clarity and developer experience. The new documentation is well-structured, detailed, and accurately reflects the functionality of each function, including arguments, return values, exceptions, and usage examples. These changes are a valuable contribution to the project's documentation standardization effort. The review did not identify any issues of high or critical severity.