Support **DictConfig hparam serialization (#2519)
Support **DictConfig hparam serialization (#2519)

* change to OmegaConf API

Co-authored-by: Omry Yadan <9682fabc61a765e30122846147d3b908ad50e2c2@fb.com>

* Swapped Container for OmegaConf sentinel; Limited ds copying

* Add Namespace check.

* Container removed. Pass local tests.

Co-authored-by: Omry Yadan <9682fabc61a765e30122846147d3b908ad50e2c2@fb.com>
williamFalcon/pytorch-lightning,williamFalcon/pytorch-lightning