fix(setup): Add torch as install_requires for the installation of torch_scatter

[zigzagcai]

Dear author,

Hello, thanks for your awesome work!
We are trying to integrate torch_scatter into the dependencies in our framework, but we found pip install . would throw errors during installation process.
So we add torch as the install_requires of torch_scatter. Hope it helps!

Related issues:
#365

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.20%. Comparing base (140d3ad) to head (fdbfaf6).
Report is 6 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #451      +/-   ##
==========================================
+ Coverage   97.23%   98.20%   +0.97%     
==========================================
  Files          10       10              
  Lines         217      223       +6     
==========================================
+ Hits          211      219       +8     
+ Misses          6        4       -2     

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