[doc] Add grad context manager doc to toplevel torch module. (#33877)
Summary:
fixes https://github.com/pytorch/pytorch/issues/32014
Pull Request resolved: https://github.com/pytorch/pytorch/pull/33877
Differential Revision: D20141801
Pulled By: albanD
fbshipit-source-id: bac713382a71666dd5e2499f710c51a55cc579ba
