Fake Tensor Docs (#101882)
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### <samp>🤖 Generated by Copilot at 75f33ae</samp>
> _Fake tensors help_
> _compile and optimize code_
> _`PT2` in autumn_
Thanks @ezyang
Pull Request resolved: https://github.com/pytorch/pytorch/pull/101882
Approved by: https://github.com/eellison, https://github.com/ezyang
