Fix a bad merge from D24154657 - second try
Fix a bad merge from D24154657 - second try

Summary: This removes a line that reset the dummy batch in begin_epoch. In actuality the batch is reset in get_{train,valid}_iterator, and this line was resetting it to None unnecessarily.

Reviewed By: joshim5, jhcross

Differential Revision: D24157057

fbshipit-source-id: 59ac68327094ceff70f66d7b471fa810997fe84e

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq