[TB] Add support for hparam domain_discrete
Summary: Add support for populating domain_discrete field in TensorBoard add_hparams API
Test Plan: Unit test test_hparams_domain_discrete
Reviewed By: edward-io
Differential Revision: D22227939
fbshipit-source-id: d2f0cd8e5632cbcc578466ff3cd587ee74f847af
