[DistributedSampler] Only create torch.generator and seed when shuffling (#37604)
Summary:
We don't need to create `torch.Generator()` and seed it if we are not shuffling.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/37604
Differential Revision: D21346167
Pulled By: rohan-varma
fbshipit-source-id: 6ed560d236bc5c026a7d321755ddc02a29db1604
