Relax sampler check in BatchSampler (#38403)
Summary:
Since the check was added in https://github.com/pytorch/pytorch/pull/6249, one can not pass an iterable as a sampler to the data loader anymore, which was a very handy feature (e.g., https://github.com/pytorch/pytorch/issues/1337). I think the check should be removed for two-fold reasons:
1. It is too strict. There is no reason that it should not be a general iterable.
2. It is inconsistent. In `DataLoader` (the main place where people use samplers), you can pass a general iterable as `batch_sampler` but not `sampler` due to this check.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38403
Differential Revision: D21555958
Pulled By: soumith
fbshipit-source-id: c7267bb99a31edd8f2750689205d6edc5dab5cff
