Update namespaces inside torch.utils.data to the latest.
# What does this PR do?

Address #13036 .
1. Replace `torch.utils.data.dataset` with `torch.utils.data`
2. Replace `torch.utils.data.sampler` with `torch.utils.data`
3. Replace `torch.utils.data.dataloader` with `torch.utils.data`

## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

@sgugger 

Is this compatible with older versions of PyTorch? Given the issue history this looks good though<|||||>@patrickvonplaten Yes, it's backward compatible with PyTorch 1.2.0+, which meets transformers' PyTorch requirements: 1.3.1+.

Ref:
https://pytorch.org/docs/1.2.0/data.html
https://github.com/pytorch/pytorch/blob/v1.2.0/torch/utils/data/__init__.py