Migrate `nonzero` from the TH to Aten (CPU)

[VitalyFedyunin]

Porting TH operators is essential for code simplicity and performance reasons.

Porting guides and Q&A are available in umbrella issue: #24507

Feel free to add @VitalyFedyunin as a reviewer to get a prioritized review.

cc @ezyang @gchanan @zou3519 @bdhirsh @jbschlosser

[anjali411]

making it high priority because it's useful for complex tensors