[NVFuser] Add environment variable to force disable NVFuser
PYTORCH_JIT_USE_NNC_NOT_NVFUSER=1 will force NVFuser to be disabled,
regardless of other environment variables or values set at runtime. It
will be used for guarding certain parts of the internal rollout.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/77168
Approved by: https://github.com/jjsjann123, https://github.com/eellison
