[caffe2] update load_save_test.py to also verify the chunking behavior (#53401)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/53401
This is a reland of D26641599 (https://github.com/pytorch/pytorch/commit/cd9ac54ea75d4a530b6b9dccaaf1167cabafc789) after rebasing onto D26802576 (https://github.com/pytorch/pytorch/commit/f595ba1bae9a75c6403e4624e7294b5635acd40c).
Add some small utility functions to read the blob names back from the minidb
file so that we can verify how many chunks were written for each blob.
ghstack-source-id: 123567033
Test Plan: buck test caffe2/caffe2/python/operator_test:load_save_test
Reviewed By: mraway
Differential Revision: D26853942
fbshipit-source-id: 0b45078fdd279f547752c8fdb771e296374a00da
