pytorch quantization: document the custom module APIs (#67449)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/67449
Adds a description of what the current custom module API does
and API examples for Eager mode and FX graph mode to the main
PyTorch quantization documentation page.
Test Plan:
```
cd docs
make html
python -m http.server
// check the docs page, it renders correctly
```
Reviewed By: jbschlosser
Differential Revision: D31994641
Pulled By: vkuzo
fbshipit-source-id: d35a62947dd06e71276eb6a0e37950d3cc5abfc1
