Add LM capabilities to TFTransfoXLLMHead
# 🚀 Feature request

It is currently not possible to generate language with the TFTransfoXLLMHeadModel because 
the `lm_head` is not implemented (see https://github.com/huggingface/transformers/blob/908fa43b543cf52a3238129624f502240725a6a6/src/transformers/modeling_tf_transfo_xl.py#L745)

Doing: 

```
from transformers import TFTransfoXLLMHeadModel
model = TFTransfoXLLMHeadModel.from_pretrained('transfo-xl-wt103') 
model(tf.convert_to_tensor([[1, 5, 8]])) 
```

currently leads to an error. 

## Motivation

Pytorch has TransfoXLLMHead model working - TF should has well.

## Your contribution

@LysandreJik , @thomwolf  I could implement it if needed.
