add quantized groupnorm operator (#36835)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/36835
Adds a quantized groupnorm operator. We reuse most of the layernorm
kernel, modifying it to be able to perform channel-wise scaling.
Benchmark results: the quantized layer is between 6x to 15x faster
from fp to q, depending on input shapes
(full results:
https://gist.github.com/vkuzo/db67623232415382dabff6c8923124e9)
Test Plan:
```
python test/quantization/test_quantized.py TestQuantizedOps.test_group_norm
python test/quantization/test_quantized.py TestQuantizedOps.test_qlayer_norm
```
Numerics are nearly equivalent, with the only difference documented
in the test case. The difference is the same type as with quantized
layernorm. Making numerics equivalent is possible but will sacrifice
speed.
Imported from OSS
Differential Revision: D21107926
fbshipit-source-id: 80e87e9e2c71310bc28c3d114c88de428819cb45
