Add Session-based Recommendations With Recurrent Neural Networks (GRU4Rec) model

[lthoang]

Description

This is an adaptation of an official implementation of GRU4Rec.

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 please also consider adding model into the docs

[tqtg]

LGTM

[tqtg]

@lthoang @hieuddo just curious whether we obtain comparable results to the original implementation on the same dataset?

[lthoang]

@lthoang @hieuddo just curious whether we obtain comparable results to the original implementation on the same dataset?

To be able to compare with original implementation, we need to further add an evaluation option for evaluating every next item in test sequence.
I have mentioned in #543 (comment)

@tqtg Should we also include that in this PR, or create a separate PR?

[tqtg]

@lthoang let have this merged and we can work on another PR for that evaluation feature