add tie_weights()

mosaic_gpt.py

@@ -242,6 +242,9 @@ class MosaicGPT(PreTrainedModel):
     
     def set_output_embeddings(self, new_embeddings):
         self.lm_head = new_embeddings    
+    
+    def tie_weights(self):
+        self.lm_head.weight = self.transformer.wte.weight
 
     def forward(
             self,