Summary:
This diff is introducing abstractions for parameter sharing for all the
parameters, that are created through new create_param syntax.
Possible use-cases of this parameters sharing:
1. Share params within RNN interface.
2. Some complicated models that might share some of the branches.
3. TODO (next diff): Cross-model parameter sharing.
Reviewed By: salexspb
Differential Revision: D5160935
fbshipit-source-id: c6d40a5ed7ead240cd7db0eb69de6dc5f505b05a
