Cannot import functions and classes directly

[parhamfh]

First of all, thank you for a nice library for rankers 😺

I found out you cannot import as you would expect from the documentation. This is not specific to the metrics module, but to illustrate my point I'm using https://www.tensorflow.org/ranking/api_docs/python/tfr/keras/metrics/MRRMetric

>>> import tensorflow_ranking
>>> tensorflow_ranking.keras.metrics.MRRMetric
<class 'tensorflow_ranking.python.keras.metrics.MRRMetric'>
>>> from tensorflow_ranking.keras.metrics import MRRMetric
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ModuleNotFoundError: No module named 'tensorflow_ranking.keras'

Seeing as how the docs specify that the MRRMetric should be available to import on this path, this is a bit misleading. You can of course import from tensorflow_ranking.python.keras.metrics but that is not immediately obvious from the documentation.

Version: 0.5.1.dev

🙇🏽