fix memory leak in pyspeech
fix memory leak in pyspeech

Summary:
Recently pyspeech users have reported memory leaks, where the RAM used by the job would spike up at the start of every epoch. Kritika and I debugged this and we ran bisect:

hg bisect -c "./script.sh" (script.sh: P143079305, training json: P143079313)

This identified my diff in D23443012 (https://github.com/pytorch/fairseq/commit/e171c8d86a939cf4ebc483cd649bee1935379771) as the culprit. I then found out it was specifically the line where we wrap the torch dataloader with itertools.islice that causes issues.

Differential Revision: D23817986

fbshipit-source-id: 22eee7d384944febddc197bb734e9db79d73e6ea

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq