Change is_variable() to check existence of AutogradMeta, and remove is_variable_ (#19139)
Summary:
Currently, a TensorImpl's `is_variable_` is true if and only if the TensorImpl has AutogradMeta. This PR unifies these two concepts by removing `is_variable_` and change `is_variable()` to check existence of AutogradMeta instead.
Removing `is_variable_` is part of the work in Variable/Tensor merge.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19139
Differential Revision: D14893339
Pulled By: yf225
fbshipit-source-id: ceb5e22c3c01f79b5d21d5bdbf4a7d1bc397796a
