Epoch duration may be based on batches_per_epoch or duration in minutes
Epoch duration may be based on batches_per_epoch or duration in minutes

Summary:
Updating the reader Limiter to identify an epoch end either based on
batches_per_epoch or epoch_duration_len.
I am basically addressing the review comment of D6299602 where I was asked to
break that diff into 2 smaller diffs.
This is Part 1 of the diff D6299602 i.e. making the multi-reader capable of identifying
epoch end either based on batches_per_epoch or based on epoch_duration_minutes

Reviewed By: azzolini

Differential Revision: D6379955

fbshipit-source-id: b8f8e396f515c898ad2f9ee900ec8fad055306b0

davinwang/caffe2,davinwang/caffe2,xzturn/caffe2,Yangqing/caffe2,davinwang/caffe2,davinwang/caffe2,xzturn/caffe2,caffe2/caffe2,xzturn/caffe2,Yangqing/caffe2,xzturn/caffe2,Yangqing/caffe2,Yangqing/caffe2,Yangqing/caffe2,xzturn/caffe2,davinwang/caffe2