Move //xplat/caffe2:caffe2_serialize to shared build structure
Summary: This is a first step to migrate xplat targets to shared build structure. Eventually both xplat buck and open source bazel targets will be generated from shared build.bzl.
Test Plan: Should be no-op, rely on CI.
Reviewed By: malfet
Differential Revision: D35270004
Pull Request resolved: https://github.com/pytorch/pytorch/pull/75847
Approved by: https://github.com/linbinyu
