Implement Sampler to create minibatches

[vimalmanohar]

Sampler to create minibatches based on the length of the utterances so that similar length utterances are together.
This needs to be moved to a separate module outside the loss function module.

It has access to a list of utterances sorted by length so that same-size batches can be created easily.

• Create batches of appropriate size.
• Return randomly sampled batches.
• Random sampling within a batch.
• Different length utterances should have different minibatch sizes (This might be different from the usual approach which uses same minibatch size. We have to use the kaldi approach for this).

[Note from Dan: we could consider padding with silence or speed-perturbing (like Hossein does) to make sure the utterance lengths are not all distinct. Also, I’d like to be able to support chunks of utterances, but this is not a hurry right now. You may have to have a mechanism to use different minibatch sizes for different utterance lengths, if you have very variable utterance lengths.]