[caffe2] update load_save_test.py to also verify the chunking behavior
Summary:
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.
Test Plan: buck test caffe2/caffe2/python/operator_test:load_save_test
Reviewed By: mraway
Differential Revision: D26641599
fbshipit-source-id: bccb0af157d85e585e95bc7be61c4584fba3cb04
