Revert D22396896: [pytorch][PR] run single-threaded gradgradcheck in test_nn
Test Plan: revert-hammer
Differential Revision:
D22396896 (https://github.com/pytorch/pytorch/commit/dac63a13cbe105c81db4539147687d593704fe83)
Original commit changeset: 3b247caceb65
fbshipit-source-id: 90bbd71ca5128a7f07fe2907c061ee0922d16edf
