[DtoH sync] set scheduler begin index in SD3 T2I

[laradipupo]

Description

Sets the scheduler's begin index to 0 before the denoising loop in SD3 T2I, eliminating the implicit GPU→CPU synchronization point from index_for_timestep.item(). This enables full torch.compile graph capture.

Follows the established pattern from PR #11696 (already deployed in Flux, Flux2, Wan, Z-Image, OVIS-Image, and SDXL).

Changes

• src/diffusers/pipelines/stable_diffusion_3/pipeline_stable_diffusion_3.py: Added canonical set_begin_index(0) block with hasattr guard before the denoising loop.

Verification

• Fast tests pass: pytest tests/pipelines/stable_diffusion_3/ -x -v
• make style && make fix-copies && make quality clean

This is commit 1 of 6 per the Phase 1 plan.

[github-actions]

Hi @laradipupo, thanks for the PR! It does not appear to link an issue it fixes. If this PR addresses an existing issue, please add a closing keyword (e.g. Fixes #1234) to the PR description so the issue is linked. See the contribution guide for more details. If this PR intentionally does not fix a tracked issue, a maintainer can add the no-issue-needed label to silence this reminder.