Remove 6px horizontal padding in span.pre

[garymm]

Sphinx 3.x puts class signatures inside <span class="pre"> tags,
and this 6px horizontal padding was making it look bad.
Seen screenshots below.

I'm not sure why this padding was added in the first place, so it's possible this change makes something worse, but from a few minutes of poking around I don't see anything that looks bad.

Part of addressing general sphinx 3 support: #115.

Before:

After:

[NicolasHug]

Thanks a lot for the PR @garymm

Would it be possible to also take care of type annotations? We have a bunch in the domains libraries, and if pytorch ever introduces them, their docs will be broken as well. Taking the example from #115, this PR transforms

into

which unfortunately still looks broken

[albanD]

LGTM thanks for fixing this!

@brianjo is there anything we need to do before merging a PR here?

[NicolasHug]

@albanD @brianjo just FYI, by merging this and keeping sphinx 3 in pytorch, you're taking the risk that the docs will look broken as soon as pytorch starts adding type annotations.

@brianjo would it be possible to move #115 forward to avoid such issues?