add `torch.testing` to docs (#57247)
Summary:
Redo of https://github.com/pytorch/pytorch/issues/56373 out of stack.
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To reviewers: **please be nitpicky**. I've read this so often that I probably missed some typos and inconsistencies.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/57247
Reviewed By: albanD
Differential Revision: D28247402
Pulled By: mruberry
fbshipit-source-id: 71142678ee5c82cc8c0ecc1dad6a0b2b9236d3e6
