Add thread-local guard: at::AutoNonVariableTypeMode (#15939)

Add thread-local guard: at::AutoNonVariableTypeMode (#15939)

Summary:
This PR adds thread-local guard (`at::AutoNonVariableTypeMode`) to make sure that in VariableType.cpp the operations on baseType still dispatch to non-Variable type, even if the parameters will become Variables after the Tensor/Variable merge. We achieve this by making `legacyTensorType()` and `getType()` check the `at::AutoNonVariableTypeMode` guard to decide whether to return non-Variable type for a variable.

This is part of the VariableImpl/TensorImpl merge work: https://github.com/pytorch/pytorch/issues/13638.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15939

Reviewed By: ezyang

Differential Revision: D13640980

Pulled By: yf225

fbshipit-source-id: d12c2543822958558d7d70d36c50999a5eb8783f