BAMultiShapes dataset

[steveazzolin]

This PR integrates the new dataset proposed in Azzolin et al.
Specifically:

• BAMultiShapesDataset loads the original dataset as defined in the paper
• BAMultiShapesGraph is a generator to create a new instance of the dataset

My original goal was to incorporate BAMultiShapesGraph into ExplainerDataset. However, the code of such Dataset seems tailored to generate node classification explainability datasets, and I found that including it would result in major modifications to its original implementation, which I was not particularly liking from a modular-code point of view. Hence, I proposed an ad-hoc Generator

[codecov]

Codecov Report

Merging #6541 (ab8db5a) into master (743a6b8) will not change coverage.
The diff coverage is n/a.

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  Misses       3229     3229           

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[rusty1s]

Thank you! Can we add the graph generator code in a separate PR? I am not totally sure if that should be done as a GraphGenerator - the underlying implementation should expect a GraphGenerator and a list of Motifs as input to generate the dataset. WDYT?