Adds CaptumExplainer class

[RBendias]

#6375

[codecov]

Codecov Report

Merging #6383 (46266c8) into master (be73d96) will increase coverage by 0.00%.
The diff coverage is 91.66%.

❗ Current head 46266c8 differs from pull request most recent head 3f82af2. Consider uploading reports for the commit 3f82af2 to get more accurate results

@@           Coverage Diff           @@
##           master    #6383   +/-   ##
=======================================
  Coverage   84.63%   84.64%           
=======================================
  Files         394      395    +1     
  Lines       21467    21490   +23     
=======================================
+ Hits        18169    18190   +21     
- Misses       3298     3300    +2     

Impacted Files	Coverage Δ
torch_geometric/explain/algorithm/base.py	93.33% <ø> (ø)
...ch_geometric/explain/algorithm/captum_explainer.py	90.47% <90.47%> (ø)
torch_geometric/explain/algorithm/__init__.py	100.00% <100.00%> (ø)
torch_geometric/explain/config.py	100.00% <100.00%> (ø)

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[rusty1s]

We can merge this with test_captum.py and captum.py since Vid's PR just landed.