give a clear message if model.get_ntypes()<data.get_ntypes() (deepm…

…odeling#1016)

Fix deepmodeling#1015.

deepmd/train/trainer.py

@@ -295,7 +295,16 @@ def build (self,
             # Usually, the type number of the model should be equal to that of the data
             # However, nt_model > nt_data should be allowed, since users may only want to 
             # train using a dataset that only have some of elements 
-            assert (self.ntypes >= data.get_ntypes()), "ntypes should match that found in data"
+            if self.ntypes < data.get_ntypes():
+                raise ValueError(
+                    "The number of types of the training data is %d, but that of the "
+                    "model is only %d. The latter must be no less than the former. "
+                    "You may need to reset one or both of them. Usually, the former "
+                    "is given by `model/type_map` in the training parameter (if set) "
+                    "or the maximum number in the training data. The latter is given "
+                    "by `model/descriptor/sel` in the training parameter." % (
+                        data.get_ntypes(), self.ntypes
+                ))
             self.type_map = data.get_type_map()
             self.batch_size = data.get_batch_size()
             self.model.data_stat(data)