run single-threaded gradgradcheck in test_nn (#40999)
Summary:
Most time-consuming tests in test_nn (taking about half the time) were gradgradchecks on Conv3d. Reduce their sizes, and, most importantly, run gradgradcheck single-threaded, because that cuts the time of conv3d tests by an order of magnitude, and barely affects other tests.
These changes bring test_nn time down from 1200 s to ~550 s on my machine.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40999
Differential Revision: D22396896
Pulled By: ngimel
fbshipit-source-id: 3b247caceb65d64be54499de1a55de377fdf9506
