Move VariableInfo into its own file to avoid circular dependency (#120732)
VariableInfo is used by both `custom_function.h` (in a templated class) and `compiled_autograd.h` (in a class with some templated methods). Another way could have been to make a `compiled_autograd.cpp` and forward declare VariableInfo, but this VariableInfo was also being used in other nodes like PyNode so it felt cleaner to do it this way.
Differential Revision: [D54287007](https://our.internmc.facebook.com/intern/diff/D54287007)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/120732
Approved by: https://github.com/jansel
