Add missing token classification for XLM
The current [modeling_xlm.py](https://github.com/huggingface/transformers/blob/master/src/transformers/modeling_xlm.py) did not have alike `ForTokenClassification` class like others, which helps for NER task comparison across all existing models.

now `XLMForTokenClassification` can be called via:
```python
from transformers import XLMForTokenClassification

model = XLMForTokenClassification.from_pretrained('xlm-mlm-100-1280')

```