quant docs: add and clean up GroupNorm (#40343)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40343
Cleans up the quantized GroupNorm docstring and adds it to quantization docs.
Test Plan: * build on Mac OS and inspect
Differential Revision: D22152635
Pulled By: vkuzo
fbshipit-source-id: 5553b841c7a5d77f1467f0c40657db9e5d730a12
