Expose enablement of TensorExpr fuser as env variable (#35341)
Summary:
This commit allows one to use an environment variable to enable the fuser in torch/csrc/jit/tensorexpr/
```
PYTORCH_TENSOREXPR=1 python benchmark.py
```
This commit also changes the registration to happen by default, removing the requirement for the python exposed "_jit_register_tensorexpr_fuser"
Pull Request resolved: https://github.com/pytorch/pytorch/pull/35341
Reviewed By: ZolotukhinM
Differential Revision: D20676348
Pulled By: bwasti
fbshipit-source-id: 4c997cdc310e7567c03905ebff72b3e8a4c2f464
