Created by: sidgoyal78

This PR basically implements a different approach for issue #7722 (closed) , and is based on the suggestions in PR #7780.

Things added:

• load_combine_op
• save_combine_op
• A unit test for saving and loading multiple LoDTensors
• A unit test for saving and loading 1 LoDTensor (so as to verify that it performs the function of the original load_op and save_op as well).

TODO: To remove the original load_op and save_op and rename these.