Added subgraph_mode

[beabevi]

Use as adjacency matrix the subgraph around nodes having
ground-truth hints that changed from the previous iteration.

Run with the star subgraphs by using
python3 -m clrs.examples.run -algorithm dfs --processor_type gatv2 --hint_mode encoded_decoded_nodiff --hint_teacher_forcing_noise 0.5 --subgraph_mode stars

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