add quantized instancenorm operator (#36847)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/36847
Adds a quantized instancenorm operator, which can reuse most of
groupnorm's logic.
Benchmarking shows that the quantized version is about 10x faster than
floating point for equivalent input sizes
(https://gist.github.com/vkuzo/2f230e84d26f26cc6030afdbfbc8e7f0)
Test Plan:
```
python test/quantization/test_quantized.py TestQuantizedOps.test_instance_norm
```
Imported from OSS
Differential Revision: D21107925
fbshipit-source-id: 6bacda402f0eb9857bc8f9a5cf8ef306150613d4
