Try to access sys.stdin.fileno() only at runtime and not during import (#553)

Summary:
Accessing sys.stdin.fileno() raises an error in multiple contexts
(pytest, joblib, jupyter...).
Thus accessing it at the top level of the file can cause other scripts
to crash when they import fairseq.
This is why it is moved inside the method of MultiprocessingPdb to only
be accessed at runtime if needed.

See  Issue #517
Pull Request resolved: https://github.com/pytorch/fairseq/pull/553

Differential Revision: D14309284

Pulled By: myleott

fbshipit-source-id: 6ca36f2053a86ebc02e2d6f025459c6a78c592e7