Improve `Tensor` type hints (#28578)
Summary:
I've typed some attributes from https://github.com/pytorch/pytorch/blob/ee920b92c4d52c3481ce4fa1594893fde8ccdd93/torch/csrc/autograd/python_variable.cpp#L490 that were not included in the stubs so that MyPy will be aware of them. I made sure to only add those attributes that are mentioned somewhere in the documentation. If there are attributes mentioned in the documentation that are not meant to be part of the public API (or the opposite), please let me know. I've also made sure that attributes that can't be set are typed as read-only properties. If setting `dtype`, `shape`, `device` or `names` directly is not part of the public API, let me know and I'll make them properties as well.
I've also added `__len__`, `__iter__` and `__contains__`, which means MyPy will no longer complain about `len(t)`, `t1 in t2` and `for t1 in t2`.
Shameless plug: I have another typing-related PR here that needs review: https://github.com/pytorch/pytorch/pull/27445
Fixes https://github.com/pytorch/pytorch/issues/28457
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28578
Reviewed By: lerks
Differential Revision: D18113954
Pulled By: fmassa
fbshipit-source-id: 0b69a2966d22054d8d87392f19ec5aa3918773bc
