[caffe2] support serializing float data as bfloat16 (#53735)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/53735
Add an option to BlobSerializationOptions to request that float data be
serialized as bfloat16. This reduces the serialized data size at the expense
of some loss in precision.
ghstack-source-id: 124317910
Test Plan: Included a new unit test.
Reviewed By: mraway
Differential Revision: D26658205
fbshipit-source-id: 74521ed161059066355a3f208488ed01a344dbb5
