Feature/item net

[In48semenov]

Description

Added embeddings categorical item features to ItemNetBlock, Constructed Item Net for embeddings items ids and features. Also added method to retrieve categorical features from SparseFeatures.

Type of change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Optimization

How Has This Been Tested?

Before submitting a PR, please check yourself against the following list. It would save us quite a lot of time.

• Have you read the contribution guide?
• Have you updated the relevant docstrings? We're using Numpy format, please double-check yourself
• Does your change require any new tests?
• Have you updated the changelog file?

[In48semenov]

TODO:

• [x] Tutorial notebook for SasRec model with categorical features
• [x] DocString

[blondered]

We also need to update interface in example.ipynb before merge.