Fix ImplicitCF failing if not using stratified split #2024

[daviddavo]

Description

Added a variable called n_users_in_train to allow ImplicitCF to work even if a user is in test but not in train.

Related Issues

• 2024

References

Checklist:

• [x] I have followed the contribution guidelines and code style for this project.
• [ ] I have added tests covering my contributions.
• [ ] I have updated the documentation accordingly.
• [x] I have signed the commits, e.g. git commit -s -m "your commit message".
• [x] This PR is being made to staging branch AND NOT TO main branch.