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
