postprocess methods will take the output of _forward and turn it into the final output that were decided
earlier.
_sanitize_parameters exists to allow users to pass any parameters whenever they wish, be it at initialization
time pipeline(...., maybe_arg=4) or at call time pipe = pipeline(...); output = pipe(...., maybe_arg=4).
The returns of _sanitize_parameters are the 3 dicts of kwargs that will be passed directly to preprocess,
_forward and postprocess. Don't fill anything if the caller didn't call with any extra parameter. That
allows to keep the default arguments in the function definition which is always more "natural".
A classic example would be a top_k argument in the post processing in classification tasks.
```python
pipe = pipeline("my-new-task")
pipe("This is a test")
[{"label": "1-star", "score": 0.8}, {"label": "2-star", "score": 0.1}, {"label": "3-star", "score": 0.05}
