Fix formatting on cumsum.cpp (#81107)
Summary:
Fixed formatting using clang-format:
```clang-format -style=file cumsum.cpp```
Test Plan: Compile time change only. Test not needed.
Reviewed By: kirklandsign
Differential Revision: D37703962
Pull Request resolved: https://github.com/pytorch/pytorch/pull/81107
Approved by: https://github.com/kirklandsign
