[NVFuser] environment variable to turn nvfuser on or off (#76485)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/76485
Adds an environment variable `PYTORCH_JIT_ENABLE_NVFUSER` for
controlling whether or not nvfuser is enabled. This required changing
the PassManager behavior to support the case where nvfuser gets enabled
by default when PYTORCH_JIT_ENABLE_NVFUSER=1.
Previously the solution for turning nvfuser on or off was to use the
PassManager to register or un-register the pass. That works fine if the
pass starts of _disabled_, but causes issues once we try to enable the
pass by default.
The main issue with enabling by default is with the validation check to
see whether NVFuser can be turned on. The check relies on
at::globalContext().hasCUDA(), which requires CUDAHooks to be registered
before hasCUDA() wil work correctly. At static initialization time it's
difficult to ensure that CUDAHooks will be registered _before_ we
attempt to register the nvfuser pass. In OSS it worked fine, but in
internal builds it would fail on ROCm builds.
To fix this, we switch the control of NVFuser enablement to a check in
the pass. i.e. previously, we enabled/disabled nvfuser by registering or
de-registering the pass in pass manager; now, the pass is always
registered in pass manager, and enablement is done by a check within the
nvfuser pass.
Remaining TODO: Connect this with NNC so that in cases where NNC is
available but not NVFuser (i.e. on AMD gpus), NNC can be turned on
automatically.
Test Plan: Imported from OSS
Reviewed By: ejguan
Differential Revision: D35982618
Pulled By: davidberard98
fbshipit-source-id: fd5b76bc0b8c8716c96fdc04bebfb15026a7ef60
(cherry picked from commit ff14603ff5ac8d9b6c749c4f111f4a8be8023b7f)
