update README in metis and update RuntimeError info

docs/HowToChooseTuner.md

@@ -221,7 +221,8 @@ Metis belongs to the class of sequential model-based optimization (SMBO), and it
 *  It finds the global optimal point in the Gaussian Process space. This point represents the optimal configuration.
 * It identifies the next hyper-parameter candidate. This is achieved by inferring the potential information gain of exploration, exploitation, and re-sampling.
 
-Note that the only acceptable types of search space are `choice`, `quniform`, `uniform` and `randint`.
+Note that the only acceptable types of search space are `choice`, `quniform`, `uniform` and `randint`. We only support 
+numerical `choice` now. More features will support later.
 
 More details can be found in our paper: https://www.microsoft.com/en-us/research/publication/metis-robustly-tuning-tail-latencies-cloud-systems/
 

src/sdk/pynni/nni/metis_tuner/metis_tuner.py

@@ -132,10 +132,15 @@ def update_search_space(self, search_space):
                     self.x_types[idx] = 'range_continuous'
                 elif key_type == 'choice':
                     self.x_bounds[idx] = key_range
+
+                    for key_value in key_range:
+                        if not isinstance(key_value, (int, float)):
+                            raise RuntimeError("Metis Tuner only support numerical choice.")
+
                     self.x_types[idx] = 'discrete_int'
                 else:
-                    logger.info("Metis Tuner doesn't support this kind of variable.")
-                    raise RuntimeError("Metis Tuner doesn't support this kind of variable.")
+                    logger.info("Metis Tuner doesn't support this kind of variable: " + str(key_type))
+                    raise RuntimeError("Metis Tuner doesn't support this kind of variable: " + str(key_type))
         else:
             logger.info("The format of search space is not a dict.")
             raise RuntimeError("The format of search space is not a dict.")