Bipartite dense_to_sparse support

[rusty1s]

Fixes #6507

[codecov]

Codecov Report

Merging #6546 (a284fa3) into master (2b9c633) will decrease coverage by 1.74%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #6546      +/-   ##
==========================================
- Coverage   87.55%   85.82%   -1.74%     
==========================================
  Files         412      412              
  Lines       22821    22780      -41     
==========================================
- Hits        19982    19552     -430     
- Misses       2839     3228     +389     

Impacted Files	Coverage Δ
torch_geometric/utils/sparse.py	100.00% <100.00%> (ø)
torch_geometric/nn/models/dimenet_utils.py	0.00% <0.00%> (-75.52%)	⬇️
torch_geometric/nn/models/dimenet.py	17.84% <0.00%> (-49.66%)	⬇️
torch_geometric/visualization/graph.py	68.25% <0.00%> (-26.99%)	⬇️
torch_geometric/profile/profile.py	33.33% <0.00%> (-25.39%)	⬇️
torch_geometric/nn/conv/utils/typing.py	83.75% <0.00%> (-15.00%)	⬇️
torch_geometric/nn/pool/asap.py	92.00% <0.00%> (-8.00%)	⬇️
torch_geometric/nn/inits.py	67.85% <0.00%> (-7.15%)	⬇️
torch_geometric/resolver.py	82.75% <0.00%> (-6.90%)	⬇️
torch_geometric/nn/dense/linear.py	89.05% <0.00%> (-5.88%)	⬇️
... and 33 more

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