Move at::NonVariableTypeMode to TensorImpl, and check it in is_variable() (#20392)
Summary:
As part of the Variable/Tensor merge, we allow passing Tensor with AutogradMeta into ATen ops, but we want to make sure they are not treated as Variables (i.e. their `is_variable()` is false). This PR makes the necessary change to make this work.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20392
Differential Revision: D15321899
Pulled By: yf225
fbshipit-source-id: c2ab09db73c63bd71ba2d8391095f4d6b4240a9a
