fix behavior of ConcatDataset w/ negative indices (#15756)

fix behavior of ConcatDataset w/ negative indices (#15756)

Summary:
Currently, when you pass a negative index to a `Dataset` created with `ConcatDataset`, it simply passes that index to the first dataset in the list. So if, for example, we took `concatenated_dataset[-1]`, this will give us the last entry of the *first* dataset, rather than the last entry of the *last* dataset, as we would expect.

This is a simple fix to support the expected behavior for negative indices.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15756

Reviewed By: ezyang

Differential Revision: D14081811

Pulled By: fmassa

fbshipit-source-id: a7783fd3fd9e1a8c00fd076c4978ca39ad5a8a2a