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Take advantage of this to simplify the Circle CI configuration.

Don't bother with tensorboardX: it's a fallback for PyTorch < 1.1.0.

These libraries aren't always installed in the virtual environment where
isort is running. Declaring them properly avoids mixing these
third-party imports with local imports.

I don't want to consider it a dependency of transformers, but it's
usually there in local development and usually not there in CI.

I'll fix it later.

lines_after_imports = 2 is a matter of taste; I like it.