Treat torch-scatter as optional dependency

[rusty1s]

No description provided.

[codecov]

Codecov Report

Merging #6617 (740043b) into master (197d4ba) will decrease coverage by 0.02%.
The diff coverage is 94.73%.

@@            Coverage Diff             @@
##           master    #6617      +/-   ##
==========================================
- Coverage   87.43%   87.41%   -0.02%     
==========================================
  Files         422      422              
  Lines       22916    22921       +5     
==========================================
  Hits        20037    20037              
- Misses       2879     2884       +5     

Impacted Files	Coverage Δ
torch_geometric/utils/scatter.py	72.72% <66.66%> (-7.28%)	⬇️
torch_geometric/nn/conv/message_passing.py	98.91% <100.00%> (-0.01%)	⬇️
torch_geometric/nn/model_hub.py	100.00% <100.00%> (ø)
torch_geometric/typing.py	100.00% <100.00%> (ø)
torch_geometric/utils/segment.py	100.00% <100.00%> (ø)
torch_geometric/nn/conv/utils/helpers.py	0.00% <0.00%> (-80.00%)	⬇️

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