pytorch docs: add fake_quantize functions documentation (#51748)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/51748
Adding docs for `fake_quantize_per_tensor_affine` and `fake_quantize_per_channel_affine`
functions.
Note: not documenting `fake_quantize_per_tensor_affine_cachemask` and
`fake_quantize_per_channel_affine_cachemask` since they are implementation details
of `fake_quantize_per_tensor_affine` and `fake_quantize_per_channel_affine`,
and do not need to be exposed to the user at the moment.
Test Plan: Build the docs locally on Mac OS, it looks good
Reviewed By: supriyar
Differential Revision: D26270514
Pulled By: vkuzo
fbshipit-source-id: 8e3c9815a12a3427572cb4d34a779e9f5e4facdd
