Smmp batch not divisible by microbatches fix
Author: mansimane
Pull request: yes

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Fixes: 
Batch size not divisible by microbatches issue in sagemaker model parallel. Following is summary of changes:
1. Updated SequentialDistributedSampler to generate samples of multiples of batchsize.
2. Updated preds_gatherer and labels_gatherer calls to be multiple of batch size.




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Comments:
Thanks @mansimane !
I added the test and realized the implementation was not working as expected, so fixed it (I had forgotten this does not behave like the `DistributedSampler` that takes one sample every other num_replicas but sliced the indices at the beginning). If you want to have a last look to check I didn't do anything bad that would be great.

The rest of the changes are just the result of our styling scripts

Changes look good to me too. I tested with microbatch size 2.