Add docstrings

[miguelgfierro]

Description

Related Issues

#1441

Checklist:

• 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.
• This PR is being made to staging branch and not to main branch.

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[miguelgfierro]

@YanZhangADS it looks there is a small error in the unit tests:

=================================== FAILURES ===================================
______________________________ test_df_to_sparse _______________________________

test_specs = {'number_of_items': 50, 'number_of_users': 20, 'seed': 123}
python_dataset =      userID  itemID  rating  timestamp
0         1       1       5 2020-06-07
1         1       2       2 2018-10-27
2...     3 2019-03-01
998      20      49       3 2020-08-05
999      20      50       3 2018-12-03

[830 rows x 4 columns]

    def test_df_to_sparse(test_specs, python_dataset):
        # initialize the splitter
        header = {
            "col_user": DEFAULT_USER_COL,
            "col_item": DEFAULT_ITEM_COL,
            "col_rating": DEFAULT_RATING_COL,
        }
    
        # instantiate the affinity matrix
>       am = AffinityMatrix(DF=python_dataset, **header)
E       TypeError: __init__() got an unexpected keyword argument 'DF'

tests/unit/reco_utils/dataset/test_sparse.py:92: TypeError

this is different to what we fixed in this PR #1447

[anargyri]

@YanZhangADS it looks there is a small error in the unit tests:

=================================== FAILURES ===================================
______________________________ test_df_to_sparse _______________________________

test_specs = {'number_of_items': 50, 'number_of_users': 20, 'seed': 123}
python_dataset =      userID  itemID  rating  timestamp
0         1       1       5 2020-06-07
1         1       2       2 2018-10-27
2...     3 2019-03-01
998      20      49       3 2020-08-05
999      20      50       3 2018-12-03

[830 rows x 4 columns]

    def test_df_to_sparse(test_specs, python_dataset):
        # initialize the splitter
        header = {
            "col_user": DEFAULT_USER_COL,
            "col_item": DEFAULT_ITEM_COL,
            "col_rating": DEFAULT_RATING_COL,
        }
    
        # instantiate the affinity matrix
>       am = AffinityMatrix(DF=python_dataset, **header)
E       TypeError: __init__() got an unexpected keyword argument 'DF'

tests/unit/reco_utils/dataset/test_sparse.py:92: TypeError

this is different to what we fixed in this PR #1447

Sorry I changed this. Let me fix it.