add to_json_file method to configuration classes

pytorch_pretrained_bert/modeling_openai.py

@@ -225,6 +225,11 @@ class OpenAIGPTConfig(object):
         """Serializes this instance to a JSON string."""
         return json.dumps(self.to_dict(), indent=2, sort_keys=True) + "\n"
 
+    def to_json_file(self, json_file_path):
+        """ Save this instance to a json file."""
+        with open(json_file_path, "w", encoding='utf-8') as writer:
+            writer.write(self.to_json_string())
+
 
 class Conv1D(nn.Module):
     def __init__(self, nf, rf, nx):