Improve LightGCN notebook

[miguelgfierro]

Description

Related Issues

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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Codecov Report

Merging #1714 (359187a) into staging (88b1038) will increase coverage by 23.33%.
The diff coverage is n/a.

❗ Current head 359187a differs from pull request most recent head c159c9f. Consider uploading reports for the commit c159c9f to get more accurate results

@@             Coverage Diff              @@
##           staging    #1714       +/-   ##
============================================
+ Coverage     0.00%   23.33%   +23.33%     
============================================
  Files           87       87               
  Lines         9132     9132               
============================================
+ Hits             0     2131     +2131     
- Misses           0     7001     +7001     

Flag	Coverage Δ
nightly	?
pr-gate	23.33% <ø> (?)

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Impacted Files	Coverage Δ
recommenders/datasets/mind.py	0.00% <0.00%> (ø)
recommenders/datasets/movielens.py	66.37% <0.00%> (+66.37%)	⬆️
recommenders/datasets/download_utils.py	90.00% <0.00%> (+90.00%)	⬆️
recommenders/evaluation/spark_evaluation.py	87.11% <0.00%> (+87.11%)	⬆️
recommenders/models/newsrec/io/mind_iterator.py	0.00% <0.00%> (ø)
recommenders/models/newsrec/models/base_model.py	0.00% <0.00%> (ø)

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[miguelgfierro]

Added a better explanation. Please @fazamani @simonzhaoms let me know if it is clear for a first reader.

FYI @anargyri