Hparams: Return empty lists for DataProvider hyperparameter operations. (#6487)

The DataProvider abstract base class is changed to have simple
implementations of both list_hyperparameters() and
read_hyperparameters(). This is to accomodate TensorBoards' DataProvider
implementations that do not currently implement these operations. We
return empty lists in both operations.

Author: bmd3k

tensorboard/data/provider.py

@@ -394,7 +394,7 @@ def list_hyperparameters(self, ctx=None, *, experiment_ids):
         Raises:
           tensorboard.errors.PublicError: See `DataProvider` class docstring.
         """
-        pass
+        return []
 
     def read_hyperparameters(self, ctx=None, *, experiment_ids):
         """Read hyperparameter values.
@@ -411,7 +411,7 @@ def read_hyperparameters(self, ctx=None, *, experiment_ids):
         Raises:
           tensorboard.errors.PublicError: See `DataProvider` class docstring.
         """
-        pass
+        return []
 
 
 class ExperimentMetadata: