Summary:
Update several things so that sphinx compiled successfully:
conf.py (add onnx in autodoc_mock_imports and add Horizon root to sys.path)
ml/rl/caffe_utils.py (update __getattr__ such that sphnix wouldn't throw exception)
ml/rl/tensorboardX.py (update __getattr__ such that sphnix wouldn't throw exception)
Pull Request resolved: https://github.com/facebookresearch/Horizon/pull/146
Test Plan: Wait for all integration tests and unit tests pass
Reviewed By: kittipatv
Differential Revision: D16243745
Pulled By: czxttkl
fbshipit-source-id: e3b8af37ec520bde44a4a59379341bceaa981070
