Turn ONNX_ML into a proper build option. (#33424)
Summary:
The detection of the env variable ONNX_ML has been properly handled in tools/setup_helpers/cmake.py,
line 242.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/33424
Differential Revision: D20043991
Pulled By: ezyang
fbshipit-source-id: 91d1d49a5a12f719e67d9507cc203c8a40992f03
