Revert "remove dead proto_convert library (#97322)"
This reverts commit d850c33bfe3f1d1f0040738718cacb04ee449bdc.
Reverted https://github.com/pytorch/pytorch/pull/97322 on behalf of https://github.com/osalpekar due to This broke a large number of internal builds due to not being able to find proto_convert.h. See here: [D44319486](https://www.internalfb.com/diff/D44319486)
