remove reference to torch._six

[cdeepali]

What does this PR do?

Removes reference to torch._six as it has been removed in PyTorch 2.0 Reference - pytorch/pytorch#94709

Fixes #992

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[kunimatsu-tri]

Hi, can I ask the timeline for this to be merged? Since this is blocking everyone from upgrading the PyTorch version to v2.0.0, I'd like you guys to merge this as soon as possible.

[kunimatsu-tri]

Thank you for merging it!