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Summary:
As discussed with Naman earlier today. Weighted sampling with
replacement can be done on a per-epoch basis using `set_epoch()`
functionality, which generates the samples as a function of random seed
and epoch.

Additionally, `FairseqTask` needs to set the starting epoch for the
dataset at the very beginning of iterator construction.

Not yet implemented is the per-epoch iterator construction, which
is necessary to actually regenerate the batches for each epoch.
Pull Request resolved: https://github.com/fairinternal/fairseq-py/pull/861

Differential Revision: D17460687

Pulled By: jma127

fbshipit-source-id: 1c2a54f04ac96b3561c100a6fd66a9fccbe3c658