DOC: split quantization.rst into smaller pieces (#41321)
Summary:
xref gh-38010 and gh-38011.
After this PR, there should be only two warnings:
```
pytorch/docs/source/index.rst:65: WARNING: toctree contains reference to nonexisting \
document 'torchvision/index'
WARNING: autodoc: failed to import class 'tensorboard.writer.SummaryWriter' from module \
'torch.utils'; the following exception was raised:
No module named 'tensorboard'
```
If tensorboard and torchvision are prerequisites to building docs, they should be added to the `requirements.txt`.
As for breaking up quantization into smaller pieces: I split out the list of supported operations and the list of modules to separate documents. I think this makes the page flow better, makes it much "lighter" in terms of page cost, and also removes some warnings since the same class names appear in multiple sub-modules.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/41321
Reviewed By: ngimel
Differential Revision: D22753099
Pulled By: mruberry
fbshipit-source-id: d504787fcf1104a0b6e3d1c12747ec53450841da
