Move `hasPrimaryContext` to `c10::cuda` (#96800)
This method has to be accessible from `c10` to enable CUDA-12 integration.
Implemented by providing private `c10::cuda:_internal::setHasPrimaryContext` that passes the pointer to the implementation (in `torch_cuda`) back to c10.
Use global class constructor/destructor to guarantee RAII.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/96800
Approved by: https://github.com/ngimel
