Initial MUTAG Dataset Contribution

[muzz-yasir]

I used the MUTAG dataset available from DGL which caused some confusion with the heterogeneity as the raw dataset and the version on HuggingFace both appear to be a collection of homogenous graphs, whereas the DGL version is a single large heterogenous graph with unexplained node and edge types.

Hence, here I transformed the large heterogeneous graph into GLI format for any downstream use of the heterogeneity - however I'm open to also using the HuggingFace dataset in addition or instead.