Logcumsumexp for complex in CPU and CUDA (#90847)
Another PR towards solving #89205.
What's in this PR:
* The implementation of forward `logcumsumexp` for complex numbers in CPU & CUDA
* The tests on forward call of `logcumsumexp` for complex numbers
* The implementation of backward `logcumsumexp` for complex numbers
What's missing:
* The test on backward gradient of `logcumsumexp` (it complaints `RuntimeError: logcumsumexp does not support automatic differentiation for outputs with complex dtype.` and I don't know how to solve the error and I don't know where to put the test for the backward computation). If possible, I'd like this to be done in this PR.
It's really tricky to handle the edge cases here (i.e. the ones involving `inf`), but I've tried my best to put some comments explaining the reasonings of my decisions in this PR.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/90847
Approved by: https://github.com/albanD
