'TransfoXLLMHeadModelOutput' object has no attribute 'loss'
## Environment info

- `transformers` version: 4.6.1
- Platform: Linux-5.11.0-7620-generic-x86_64-with-glibc2.10
- Python version: 3.8.10
- PyTorch version (GPU?): 1.8.1 (True)
- Tensorflow version (GPU?): not installed (NA)

Models:
- Transformer XL



## To reproduce

Just to run the example from the documentation: https://huggingface.co/transformers/model_doc/transformerxl.html#transfoxllmheadmodel


Steps to reproduce the behavior:

```python
import torch
from transformers import TransfoXLTokenizer, TransfoXLLMHeadModel
tokenizer = TransfoXLTokenizer.from_pretrained('transfo-xl-wt103')
model = TransfoXLLMHeadModel.from_pretrained('transfo-xl-wt103')
inputs = tokenizer("Hello, my dog is cute", return_tensors="pt")
outputs = model(**inputs, labels=inputs["input_ids"])
loss = outputs.loss
```

## Expected behavior

I should get a loss, but an exception is thrown instead: 

AttributeError: 'TransfoXLLMHeadModelOutput' object has no attribute 'loss'


Indeed, there's an issue with the docstring! `TransfoXL` has two losses, here's the correct snippet:
```py
import torch
from transformers import TransfoXLTokenizer, TransfoXLLMHeadModel
tokenizer = TransfoXLTokenizer.from_pretrained('transfo-xl-wt103')
model = TransfoXLLMHeadModel.from_pretrained('transfo-xl-wt103')
inputs = tokenizer("Hello, my dog is cute", return_tensors="pt")
outputs = model(**inputs, labels=inputs["input_ids"])
losses = outputs.losses
```

Would you like to open a PR to fix the docstring?<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.