Cleanup weight docs

[NicolasHug]

This PR addresses my own comments from #6936 (comment)

Diff looks big but it's only a few lines in conf.py - the rest is just pure s/x/y

Before:

After:

Also renames the _weight_size key into _file_size to avoid confusion with the actual size occupied in RAM by the weights, which is also something we want to document: #5982. For the same reason I removed the file size info from the main tables, because it may be more relevant to document the weight size there instead of the file size (and there is only so much column space available).

[pmeier]

Just for my understanding: we are just changing the "keys" in the table to avoid showing the underscore which might be confusing and leaks implementation details, right?

Plus, there is a doc failure: https://app.circleci.com/pipelines/github/pytorch/vision/22527/workflows/6df95767-c7cb-4078-9c9b-4235343bdb2d/jobs/1804785?invite=true#step-108-367

[NicolasHug]

we are just changing the "keys" in the table to avoid showing the underscore

Mostly yes. I aslo renamed the (sort-of private) key from _weight_size to _file_size to avoid confusion, and removed the "Size" column from the big tables; this is because what we originally wanted to document was the weights size as in RAM, not the file size (#5982). But there's only so much info we can show in the table so I prefer waiting until we have the RAM usage info. The file size is still available in the individual models doc.

[NicolasHug]

Docs are OK now: https://output.circle-artifacts.com/output/job/ca9e66a9-b554-405e-888d-5807170f210a/artifacts/0/docs/models.html

[pmeier]

LGTM, thanks Nicolas!