Composites show as being under the __init__ module

The [built docs](https://docs.dwavequantum.com/en/latest/ocean/api_ref_preprocessing/api_ref.html) show currently like this:

This is due to the placement of the ``.. automodule:: dwave.preprocessing.composites.__init__`` directive, and it causes a problem for xrefing:

The ``objects.inv`` looks like this: