replace append with pd.concat

[gro1m]

Description

Execution of data = ImplicitCF(train, test, seed, col_user, col_item, col_rating) issues the following warning:

/anaconda/envs/ncf-microsoft-env/lib/python3.9/site-packages/recommenders/models/deeprec/DataModel/ImplicitCF.py:73: FutureWarning: The frame.append method is deprecated and will be removed from pandas in a future version. Use pandas.concat instead.
  df = train if test is None else train.append(test)

Related Issues

Checklist:

• I have followed the contribution guidelines and code style for this project.
• [Not Applicable] I have added tests covering my contributions.
• [Not Applicable] I have updated the documentation accordingly.
• This PR is being made to staging branch and not to main branch.

[miguelgfierro]

Thanks for the contribution!