Document `IValue` (#31904)
Summary:
This is a first pass attempt at documenting `IValue` to help with problems like in #17165. Most users are probably concerned with
* how to make an `IValue` that matches the input type to their graph (most of the constructors are pretty self explanatory, so as long as they are in the docs I think its enough)
* how to extract the results after running their graph (there is a small note on the behavior of `.toX()` based on confusions we've had in the past)
Preview:
https://driazati.github.io/pytorch_doc_previews/31904/api/structc10_1_1_i_value.html#exhale-struct-structc10-1-1-i-value
There are also some random CSS fixes to clean up the style.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/31904
Pulled By: driazati
Differential Revision: D19318733
fbshipit-source-id: b29dae3349d5a7ea5a3b8e09cd23f7ff8434edb4
