Disables imag for real-valued tensors (#35728)
Summary:
In NumPy, calling np.imag on a real-valued tensors returns a non-writable tensor (view) of zeros. In PyTorch we don't support non-writeable tensors (or views), so we can either return a writable tensor or error.
If we do the former, that may confuse people who try to write to the imaginary part of a real-valued tensor, and may cause a BC issue if we do support non-writable tensors. This PR errors to provide us flexibility implementation the solution we'd like in the future, while protecting users from unexpected behavior today.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/35728
Differential Revision: D20760687
Pulled By: mruberry
fbshipit-source-id: f60d445746cc75ba558804c853993d9e4621dad3
