Add trigram block by default

Similar to PRIMERA-multinews and PRIMERA-multixscience, I have found that I get ROUGE scores much closer to those reported if I add a trigram block. I am proposing this be enabled by default in the HuggingFace config.

config.json

@@ -73,6 +73,7 @@
   "model_type": "led",
   "normalize_before": false,
   "normalize_embedding": true,
+  "no_repeat_ngram_size": 3,
   "num_hidden_layers": 12,
   "output_past": false,
   "pad_token_id": 1,