Fix #3017: restore support for user modules in zip/jar files (#3018)
Fix #3017: restore support for user modules in zip/jar files (#3018)

Summary:
## What does this PR do?
Fixes https://github.com/pytorch/fairseq/issues/3017

My original implementation of the function utils.import_user_module supported external user modules wrapped in zip/jar files (python natively support modules in zip/jar files).

The new piece of code which checks for file existence breaks this functionality.
This trivial fix can solve the problem!

Pull Request resolved: https://github.com/pytorch/fairseq/pull/3018

Reviewed By: alexeib

Differential Revision: D25485611

Pulled By: myleott

fbshipit-source-id: 21667cd87e9e7a99095f8ad21d7b3bfdb547a993

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq