Fix BUILD_CAFFE2 if FBGEMM and NNPACK are not built (#45610)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/45610
Also add to the usual documentation places that this option exists.
Test Plan: Imported from OSS
Reviewed By: gmagogsfm
Differential Revision: D24058199
Pulled By: suo
fbshipit-source-id: 81574fbd042f47587e2c7820c726fac0f68af2a7
