cast scale window to int for memory efficient fp16
cast scale window to int for memory efficient fp16

Summary:
In some of my runs I found loss scale to never increase

{F358780650}

This was because scale window when using 48 GPUs by default will not be a whole integer. Lets cast it like the default fp16 optimizer.

Reviewed By: myleott

Differential Revision: D25872832

fbshipit-source-id: 7ab5c01c555dca07bda72cae3633bb4b17709a77

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq