Add QM9 dataset + QM9 custom dataset with chemical rings encoded as tensors

[martin-carrasco]

Checklist

• My pull request has a clear and explanatory title.
• My pull request passes the Linting test.
• I added appropriate unit tests and I made sure the code passes all unit tests. (refer to comment below)
• My PR follows PEP8 guidelines. (refer to comment below)
• My code is properly documented, using numpy docs conventions, and I made sure the documentation renders properly.
• I linked to issues and PRs that are relevant to this PR.

Description

Issue

Additional context

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Codecov Report

Attention: Patch coverage is 26.62722% with 124 lines in your changes missing coverage. Please review.

Project coverage is 88.17%. Comparing base (99de7cb) to head (15c784c).

Files with missing lines	Patch %	Lines
topobenchmark/data/datasets/qm9_custom_dataset.py	19.58%	115 Missing ⚠️
...data_manipulations/keep_selected_target_indices.py	45.45%	6 Missing ⚠️
...enchmark/data/loaders/graph/molecule_datasets_2.py	80.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #145      +/-   ##
==========================================
- Coverage   91.00%   88.17%   -2.84%     
==========================================
  Files         129      132       +3     
  Lines        3670     3839     +169     
==========================================
+ Hits         3340     3385      +45     
- Misses        330      454     +124     

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