hparams: expose `hparams_config_pb` from `api` (#2182)
hparams: expose `hparams_config_pb` from `api` (#2182)

Summary:
This should have been in #2174.

Test Plan:
That unit tests pass is sufficient. We don’t have demos that exercise
this endpoint. (The implementation is tested in `summary_v2_test`.)

wchargin-branch: hparams-config-pb-export
tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard