Migrate `masked_select` from TH to ATen (CUDA)

[ezyang]

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 @anjali411 @dylanbespalko as this affects complex tensor bring up

[ezyang]

This is high priority because masked_select is called when printing tensors

[kurtamohler]

Unassigned myself from this for now, in case @mingfeima wants to take it, since he's porting the CPU version in PR #33269

[mingfeima]

@kurtamohler please continue with CUDA part. I am working for Intel so mostly work on CPU part...

Also feel free to drop any comment for the CPU code as well :)