Summary:
Support different #images per worker, and 0 batch size. 5~10% slower than old implementation.

If needed, it should be used by calling the module manually - currently it cannot be controlled by a config.

Reviewed By: rbgirshick

Differential Revision: D20345669

fbshipit-source-id: ad1bf24235025ec2e6ff95635331cff52b487c1b