[NVFuser] fix force-disable flag
This prevents the std::call_once() check from erroring if:
* PYTORCH_JIT_USE_NNC_NOT_NVFUSER=1
* PYTORCH_JIT_ENABLE_NVFUSER=1
* user has not set a flag.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/77395
Approved by: https://github.com/eellison
