[M2M100Tokenizer] fix _build_translation_inputs
# What does this PR do?

The `_build_translation_inputs` method of `M2M100Tokenizer` hardcodes the `return_tensors` argument, but it's also passed as a `extra_kwargs` in translation pipeline. So the pipeline fails with the error

```TypeError: M2M100Tokenizer object got multiple values for keyword argument 'return_tensors'```

This PR adds `return_tensors` parameter to `_build_translation_inputs`. 
