[BART] Update encoder and decoder on set_input_embedding (#3501)

Co-authored-by: Ioannis Douratsos <ioannisd@amazon.com>

src/transformers/modeling_bart.py

@@ -805,6 +805,8 @@ class BartModel(PretrainedBartModel):
 
     def set_input_embeddings(self, value):
         self.shared = value
+        self.encoder.embed_tokens = self.shared
+        self.decoder.embed_tokens = self.shared
 
     def get_output_embeddings(self):
         return _make_linear_from_emb(self.shared)  # make it on the fly