Add smiles string to QM9 dataset

[hatemhelal]

The smiles string is useful for many molecular GNNs that try and learn the mapping between the SMILES representation and molecular properties. This change relies on the dataset being pre-processed with RDKit.

[codecov]

Codecov Report

Merging #7867 (2167e52) into master (e95cdaf) will not change coverage.
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[hatemhelal]

@rusty1s thanks for the speedy review as ever!

Would it be possible to also update the pre-processed dataset used as a fallback when RDKit is not installed?

Happy to help with that if I can.

[rusty1s]

Would it be possible to also update the pre-processed dataset used as a fallback when RDKit is not installed?

Yes, I can take care of that.