request to add a fidelity metric

[AslanDing]

Fidelity is a critical metric for explainable graph neural networks. In this request, we implemented robust fidelity(ICLR 24)

[codecov]

Codecov Report

Attention: Patch coverage is 95.86207% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 89.28%. Comparing base (d491f43) to head (ab64379).
Report is 33 commits behind head on master.

Files	Patch %	Lines
torch_geometric/explain/metric/robust_fidelity.py	95.83%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9016      +/-   ##
==========================================
- Coverage   89.86%   89.28%   -0.59%     
==========================================
  Files         468      469       +1     
  Lines       30048    30188     +140     
==========================================
- Hits        27003    26952      -51     
- Misses       3045     3236     +191     

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