Remove compile bottlenecks from ZImage pipeline

[hitchhiker3010]

What does this PR do?

Fixes performance issues identified by profiling ZImagePipeline with torch.profiler as part of #13401 .

What does this PR do?

Profiled ZImagePipeline (using Tongyi-MAI/Z-Image-Turbo) in both eager and torch.compile modes following the profiling guide. The Chrome traces revealed two device-to-host (DtoH) synchronization points that break asynchronous GPU execution and prevent torch.compile from yielding its full speedup.

Pipeline denoising loop: t_norm = timestep[0].item() DtoH sync

1. Inside the denoising loop, timestep[0].item() triggers a GPU→CPU sync every step to read t_norm for CFG truncation logic. Since the full timestep schedule is known before the loop begins, we precompute all t_norm values into a plain Python list before entering the loop and index into it with i.
2. This also lets us set scheduler.set_begin_index(0) upfront to avoid the DtoH sync in _init_step_index (same pattern as Avoid DtoH sync from access of nonzero() item in scheduler #11696 )

Profiling ZImagePipeline
GPU - L4
num_inference_steps - 4,
guidance_scale - 0.0 ( Guidance should be 0 for the Turbo models)

Before

The first scheduler_step took 657.8µs
Number of cudaStreamSynchronize blocks - 19

After

The first scheduler_step took 15.49 µs after this fix
Number of cudaStreamSynchronize blocks - 13
Part of #13401 .

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Who can review?

@sayakpaul @dg845

[sayakpaul]

Thanks for your PR! Can we eliminate all the cudaStreamSynchronize calls?

[hitchhiker3010]

Here are some comparison stats between commit_1 and commit_2

Metric	commit_1 eager	commit_2 eager	commit_1 compile	commit_2 compile
nonzero calls	28	4	28	4
nonzero CPU time	2091 ms	0.72 ms	3057 ms	0.49 ms
index_put calls	20	0	36	0
index_put total	4183 ms	0 ms	9172 ms	0 ms
cudaStreamSynchronize calls	13	5	13	5
cudaStreamSynchronize total	2089 ms	0.47 ms	3055 ms	0.32 ms

[hitchhiker3010]

all the trace files can be accessed here.

The cudaStreamSynchronize traces from the Denoising phase are eliminated now, the remaining 5 cudaStreamSynchronize seem to be from the text encoding phase, should we fix them too?

cc: @sayakpaul