[doc] fix autograd doc subsubsection display issue (#40582)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40582
There's a misuse in the `requires_grad` with ~~~~~~, "~~~~" is not a official section marker, change it to "^^^^^" to denote subsubsections, also fix the other places where we should use subsection "-----" instead of subsubsection "^^^^"
see https://www.sphinx-doc.org/en/master/usage/restructuredtext/basics.html#sections
Before:
<img width="712" alt="rst_before" src="https://user-images.githubusercontent.com/9443650/85789835-2226fa80-b6e4-11ea-97b6-2b19fdf324a4.png">
After:
<img width="922" alt="rst_after" src="https://user-images.githubusercontent.com/9443650/85789856-281cdb80-b6e4-11ea-925f-cb3f4ebaa2bf.png">
Test Plan: Imported from OSS
Differential Revision: D22245747
Pulled By: wanchaol
fbshipit-source-id: 11548ed42f627706863bb74d4269827d1b3450d4
