Deprecate legacy constructor `torch.Tensor()` (#54414)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/47112
This pull request is the final step in [the proposed plan](https://github.com/pytorch/pytorch/issues/47112#issuecomment-789972007) for deprecating `torch.Tensor()` constructor. Specifically, it **updates the docs and throws `TORCH_WARN_ONCE` if someone uses `torch.Tensor()`**.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/54414
Reviewed By: ailzhang
Differential Revision: D27325267
Pulled By: heitorschueroff
fbshipit-source-id: 5442572603d340b89e8cc5a886a330dd9b13550a
