LambdaMART implementation with latest master rebase

[srinikrish22]

I have implemented the LambdaMART algorithm for object ranking. This branch has the latest rebase from the current master branch of cs-ranking.

Description

I have created a class with the required core functionality of lambdamart and added ndcg metric implementation to the utils. I have added the ranker into the test_ranking and also added the ranker definition in the constants.

Motivation and Context

This change aims at adding an implementation of a new ranker for object ranking to the repository.

How Has This Been Tested?

I have added the possibility to test this ranker under the existing testing infrastructure of the cs-ranking repository.

Does this close/impact existing issues?

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.

[kiudee]

Thank you for the rebase.
Overall, the code improved a lot over the previous submission.

To finalize the submission, remember to:

• Currently, I am not able to use predict without it throwing an error. This should be fixed.
• Fix the issues remarked in the code.
• Execute the black code formatter on your files and on the functions you added. Try to avoid reformatting existing files.
• Add LambdaMART to https://github.com/kiudee/cs-ranking/blob/master/csrank/objectranking/__init__.py for direct import (from csrank import LambdaMART).

[coveralls]

Pull Request Test Coverage Report for Build 700

• 123 of 175 (70.29%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.5%) to 47.507%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
csrank/objectranking/lambdamart.py	118	170	69.41%

Totals
Change from base Build 687:	0.5%
Covered Lines:	3801
Relevant Lines:	8001

---

💛 - Coveralls

[srinikrish22]

Fixes have been made based on the suggested changes. I have also run the black code formatter.

[srinikrish22]

Thank you for the rebase.
Overall, the code improved a lot over the previous submission.

To finalize the submission, remember to:

* [ ]  Currently, I am not able to use `predict` without it throwing an error. This should be fixed.

* [ ]  Fix the issues remarked in the code.

* [ ]  Execute the [black](https://black.readthedocs.io/en/stable/) code formatter on your files and on the functions you added. Try to avoid reformatting existing files.

* [ ]  Add LambdaMART to https://github.com/kiudee/cs-ranking/blob/master/csrank/objectranking/__init__.py for direct import (`from csrank import LambdaMART`).

All the changes requested have been completed:

• Currently, I am not able to use predict without it throwing an error. This should be fixed.

• Fix the issues remarked in the code.

• Execute the black code formatter on your files and on the functions you added. Try to avoid reformatting existing files.

• Add LambdaMART to https://github.com/kiudee/cs-ranking/blob/master/csrank/objectranking/__init__.py for direct import (from csrank import LambdaMART).