Fix XLNet token type number (#4125)

The max type id is 2, but the number of ids is 3

Co-authored-by: Evan Pete Walsh <epwalsh10@gmail.com>
Co-authored-by: Dirk Groeneveld <dirkg@allenai.org>

allennlp/modules/token_embedders/pretrained_transformer_embedder.py

@@ -52,7 +52,7 @@ def get_output_dim(self):
     def _number_of_token_type_embeddings(self):
         config = self.transformer_model.config
         if isinstance(config, XLNetConfig):
-            return 2  # XLNet has hardcoded 2
+            return 3  # XLNet has 3 type ids
         elif hasattr(config, "type_vocab_size"):
             return config.type_vocab_size
         else: