Adding LM Head to Transfo-XL and first step to fixing problem with Adaptive Embeddings in TransfoXL (#3286)

* first commit

* work in progress

* make language generation task pass

* update to working version for LM

* delete print

* remove dead code

* make style

src/transformers/__init__.py

@@ -357,6 +357,7 @@ if is_tf_available():
         TFTransfoXLModel,
         TFTransfoXLLMHeadModel,
         TF_TRANSFO_XL_PRETRAINED_MODEL_ARCHIVE_MAP,
+        TFAdaptiveEmbedding,
     )
 
     from .modeling_tf_xlnet import (