Include all synthetic data referenced in GNNExplainer

[Sutongtong233]

A smaller PR for #8704

Include grid motifs and tree-based datasets in GNNExplainer.

• add torch_geometric/datasets/graph_generator/tree_graph.py
• add torch_geometric/datasets/motif_generator/grid.py
• add node label in torch_geometric/datasets/motif_generator/cycle.py (according to bug report CycleMotif lack of label, therefore do not support GNNExplainer. #8509, it is better to directly add node label since we know what the label should be, rather than be captured at else sentense in line 127 of ./torch_geometric/datasets/explainer_dataset.py)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (3fb6ff1) 89.56% compared to head (c9a0d15) 89.57%.

❗ Current head c9a0d15 differs from pull request most recent head c05c214. Consider uploading reports for the commit c05c214 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #8736   +/-   ##
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  Coverage   89.56%   89.57%           
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  Files         481      481           
  Lines       31048    31048           
=======================================
+ Hits        27809    27810    +1     
+ Misses       3239     3238    -1     

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