Ensure consistent line endings in the repository

[akihironitta]

I observed that our repo has multiple types of line endings. This PR ensures that line endings are all lf and that we never mix them in the repository.

Refs:

• https://docs.github.com/en/get-started/getting-started-with-git/configuring-git-to-handle-line-endings#per-repository-settings
• Add .gitattributes for consistent line endings in the repo pytorch_geometric#7759

No need for a changelog entry as this change impacts no users.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.40%. Comparing base (5043806) to head (55f0211).
Report is 29 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #332   +/-   ##
=======================================
  Coverage   86.40%   86.40%           
=======================================
  Files          35       35           
  Lines        1214     1214           
=======================================
  Hits         1049     1049           
  Misses        165      165           

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