[training_utils] fix: Using a non-tuple sequence for multidimensional indexing is deprecated

[HollowMan6]

What does this PR do?

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According to the warning:

UserWarning: Using a non-tuple sequence for multidimensional indexing is deprecated and will be changed in pytorch 2.9; use x[tuple(seq)] instead of x[seq]. In pytorch 2.9 this will be interpreted as tensor index, x[torch.tensor(seq)], which will result either in an error or a different result (Triggered internally at /pytorch/torch/csrc/autograd/python_variable_indexing.cpp:306.)

Should be backward compatible and safe to merge.

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[gemini-code-assist]

Code Review

This pull request correctly addresses a PyTorch deprecation warning about using non-tuple sequences for multidimensional indexing. The changes are correct and necessary for future compatibility. During the review, I found a potential bug in the _unpad_tensor function that could lead to incorrect behavior when padding_size is zero. I've added a comment with a suggested fix that also improves code clarity.