Update next-basket evaluation

[lthoang]

Description

Recent updates in base_method.py cause error when run next-basket evaluation.

This changes is required.
I have further add a new batch iteration function in next-basket dataset for convenient of getting basket items instead of just basket ids.

• A more accurate scoring function of GPTop has been updated also enhance ranking performance.

Related Issues

Checklist:

• I have added tests.
• I have updated the documentation accordingly.
• I have updated README.md (if you are adding a new model).
• I have updated examples/README.md (if you are adding a new example).
• I have updated datasets/README.md (if you are adding a new dataset).

[tqtg]

@lthoang couple of comments:

• shall we rename the iterators to align with others and the data formats? (e.g., user_basket_iter -> ub_iter). I think they're already quite meaningful and we don't have to name them very long.
• please consider adding some unit tests for this evaluation method to avoid breaking changes in the future.

[lthoang]

@tqtg I have updated the naming for the batch iterations and added some unit tests.
In addition, in GPTop model, I added an option to use extra data in UBITJson (e.g., quantity).

[tqtg]

@lthoang please have a look at some changes that I've made. If everything looks good, please consider wrapping up this PR.