Introduce BUILD_CAFFE2 flag (#43673)
Summary:
introduce BUILD_CAFFE2 flag. default to `ON`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/43673
Reviewed By: malfet
Differential Revision: D23381035
Pulled By: walterddr
fbshipit-source-id: 1f4582987fa0c4a911f0b18d311c04fdbf8dd8f0
