Allow disabling nvfuser without CUDA (#71358)
Summary:
On a CPU-only build of pytorch `torch._C._jit_set_nvfuser_enabled(False)` would throw an error (even though it is a no-op operation), with this fix:
```
>>> torch._C._jit_set_nvfuser_enabled(False)
False
>>> torch._C._jit_set_nvfuser_enabled(True)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
RuntimeError: Running CUDA fuser is only supported on CUDA builds.
>>>
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/71358
Reviewed By: eellison
Differential Revision: D33601135
Pulled By: jansel
fbshipit-source-id: c764df2fa197ce7b4f71e5df0a91cd988766e99c
(cherry picked from commit a801df93210302e918eca7134d3c0a19ac5bae5d)
