Don't run dist_autograd_fork on Python 2 (#27612)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/27612
The file imports from torch.distributed.rpc, which won't be
initialized when running on Python 2.
Test Plan: Imported from OSS
Differential Revision: D17855033
Pulled By: pietern
fbshipit-source-id: 6e6b0ca248d0512dac5a44e10e153c710cefe02c
