Move variabletype functions around (#63330)

Move variabletype functions around (#63330)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/63330

 - This is in preparation for templated/boxed autograd-not-implemented fallback
 - Make sure VariableTypeUtils does not depend on generated code
 - Lift `isFwGradDefined` into `autograd/functions/utils.cpp` so it's available to mobile builds
 - Removes `using namespace at` from VariableTypeUtils, previously we needed this for Templated version, but now its not strictly necessary but still a good change to avoid name conflicts if this header is included elsewhere in the future.

Test Plan: Imported from OSS

Reviewed By: heitorschueroff

Differential Revision: D30518573

Pulled By: soulitzer

fbshipit-source-id: a0fb904baafc9713de609fffec4b813f6cfcc000