fix pad_token_id

ID of pad token was set to -1 in the model which can lead to indexing errors.

config.json

@@ -135,7 +135,7 @@
     "output_attentions": false,
     "output_hidden_states": false,
     "output_scores": false,
-    "pad_token_id": -1,
+    "pad_token_id": 32000,
     "prefix": null,
     "problem_type": null,
     "pruned_heads": {},