memory efficient per-channel fq: use it everywhere, delete old version (#51265)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/51265
This PR is the cleanup after #51159. High level, we make the new
definition of fake_quant per channel be the definition used by autograd, but keep the old
function around as a thin wrapper to keep the user facing API the same.
In detail:
1. point fake_quantize_per_channel_affine's implementation to be fake_quantize_per_channel_affine_cachemask
2. delete the fake_quantize_per_channel_affine backward, autograd will automatically use the cachemask backward
3. delete all the fake_quantize_per_channel_affine kernels, since they are no longer used by anything
Test Plan:
```
python test/test_quantization.py TestFakeQuantize
```
Imported from OSS
Reviewed By: jerryzh168
Differential Revision: D26120957
fbshipit-source-id: 264426435fabd925decf6d1f0aa79275977ea29b
