[caffe2] expose whether FBGEMM is available to the Python code (#54274)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/54274
Some of the Python tests need to be aware of whether or not FBGEMM is
available, so expose this setting in the pybind extension.
ghstack-source-id: 124317732
Test Plan: Will use this variable in the tests on D26658205.
Reviewed By: mraway
Differential Revision: D27171780
fbshipit-source-id: 4c94144a959bf8bf0e1553b6e029e94a91794e29
