Use torch._dynamo instead of torchdynamo (#1252)
Summary:
Pull Request resolved: https://github.com/pytorch/benchmark/pull/1252
pytorch/torchdynamo got moved into pytorch/pytorch, so we should use
torch._dynamo instead of torchdynamo to get the latest version of
dynamo.
functorch was also moved into pytorch/pytorch.
Test Plan: Imported from OSS
Reviewed By: xuzhao9
Differential Revision: D40455294
Pulled By: davidberard98
fbshipit-source-id: df42a5f33853908767e7db9d8fe4ce93139060e9
