Fix `m2m_100.mdx` doc example missing `labels`
Author: Mustapha-AJEGHRIR
Pull request: yes

# What does this PR do?

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The `labels` variable is not defined, the `model_inputs` already contain this information :

```python
model_inputs.keys()           # dict_keys(['input_ids', 'attention_mask', 'labels'])
```


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Models:

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