Remove docs from flake8 (#446)

Summary:
In release branch, due to my PR #444. The lint CI is broken because I remove getting version from `torchdata` and `torchdata` is imported but not used in docs anymore.

I don't want to add more burden for ppl whoever takes in charge of release engineering in the future. I believe it's not necessary to check flake8 for docs folder.

Pull Request resolved: #446

Reviewed By: NivekT

Differential Revision: D36610568

Pulled By: ejguan

fbshipit-source-id: 3b690a9dcdd5c3c6f32a5e0aa58fa60b75dc1140

.flake8

@@ -10,3 +10,4 @@ exclude =
     ./third_party,
     *.pyi,
     ./examples/text/utils.py, # Copy from TorchText
+    docs/source/conf.py,