Set scheduler begin index in remaining FlowMatch pipelines to avoid DtoH sync (#14576)

Propagate the set_begin_index(0) fix from #11696 to HunyuanVideo, Mochi,
Lumina2, AuraFlow, and Chroma text-to-image/video pipelines. Without it,
the first scheduler.step() routes through _init_step_index() ->
index_for_timestep(), whose nonzero().item() forces a device-to-host
sync on the first denoising step. Fixes #14573.

Co-authored-by: Sayak Paul <spsayakpaul@gmail.com>