Remove compile bottlenecks from ZImage pipeline (#13461)
* [core] Remove DtoH syncs from ZImage pipeline denoising loop
* [core] Replace boolean mask indexing with torch.where in ZImage transformer
Boolean mask indexing (tensor[mask] = val) implicitly calls nonzero(),
which triggers a DtoH sync that stalls the CPU while the GPU queue drains.
Replacing it with torch.where eliminates these syncs from the transformer's
pad-token assignment.
Profiling (4-step turbo, fix_2 vs fix_1):
- Eager: nonzero CPU time drops from ~2091 ms to <1 ms; index_put eliminated
- Compile: nonzero CPU time drops from ~3057 ms to <1 ms; index_put eliminated
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Co-authored-by: Sayak Paul <spsayakpaul@gmail.com>
