fix pt bug: missing get_ntype method (#3612)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

deepmd/pt/model/model/model.py

@@ -45,3 +45,8 @@ def compute_or_load_stat(
     def get_model_def_script(self) -> str:
         """Get the model definition script."""
         return self.model_def_script
+
+    @torch.jit.export
+    def get_ntypes(self):
+        """Returns the number of element types."""
+        return len(self.get_type_map())