Remove pytorch codes in Class TFXLNetMainLayer
This PR removes pytorch codes in Class TFXLNetMainLayer.
Sorry I don't really understand how this PR removes pytorch code in `TFXLNetMainLayer` - can you explain a bit more in-detail?<|||||>@patrickvonplaten  

The codes in file modeling_tf_xlnet.py  is the XLNet Model of tensorflow implementation. So the parameter **head_mask**  should have a type of tf.Tensor or Numpy array (see Line 780 ).

But in  Lines 643-650, the parameter **head_mask** uses the methods of the pytorch, such as "expand" or "unsqueeze" which can't apply to tf.Tensor or Numpy array. 

Actually these codes are copied from modeling_xlnet.py by mistake  and should be removed.<|||||>Perfect thanks a lot! 

@LysandreJik  - the RUN_SLOW=1 tests all poss for TFXLNetMainLayer.

Good to merge for me!