Add SubgraphType with bidirectional sampling support (1/2)

[rusty1s]

No description provided.

[codecov]

Codecov Report

Merging #7199 (35ccf1d) into master (bff6405) will decrease coverage by 0.43%.
The diff coverage is 97.95%.

❗ Current head 35ccf1d differs from pull request most recent head 3b6b4e8. Consider uploading reports for the commit 3b6b4e8 to get more accurate results

@@            Coverage Diff             @@
##           master    #7199      +/-   ##
==========================================
- Coverage   92.10%   91.68%   -0.43%     
==========================================
  Files         436      436              
  Lines       23937    23980      +43     
==========================================
- Hits        22048    21985      -63     
- Misses       1889     1995     +106     

Impacted Files	Coverage Δ
torch_geometric/testing/data.py	40.62% <80.00%> (+4.91%)	⬆️
torch_geometric/sampler/base.py	98.33% <100.00%> (+0.30%)	⬆️
torch_geometric/sampler/utils.py	93.65% <100.00%> (+1.49%)	⬆️
torch_geometric/utils/coalesce.py	100.00% <100.00%> (ø)

... and 21 files with indirect coverage changes

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