[caffe2] create and register child ws in pybind (#36741)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/36741
Create child workspace that shares parent workspace's blobs. Register child workspace in registrar to enable switching into child workspace and feeding to child workspace alone.
Test Plan: numeric suite unit tests in stacked diff
Reviewed By: hx89
Differential Revision: D21055567
fbshipit-source-id: 374b12aef75a4c58452c271f8961ee156ce6c559
