Add `**kwargs` to pipeline `_decorator` call signature

[joeycarter]

Context: The main and v0.9.0-rc branches became out of sync: main currently has a **kwargs argument in the pipeline _decorator call signature, but v0.9.0-rc does not. The function argument is needed because it's referenced in the function body:

def _decorator(fn=None):  # <- **kwargs should be in the call signature
    if fn is None:
        return functools.partial(pipeline, **kwargs)
    ...

Note that we don't have good testing coverage for this particular branch of the code since all tests pass with or without the **kwargs argument in the call signature. However, we should add it in for the sake of internal consistency.

Description of the Change: Add **kwargs to pipeline _decorator call signature

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