Warn when add_noise runs before set_timesteps #15043

searched issues, related to #15021 but not the same.

add_noise before set_timesteps silently uses the wrong level on a few schedulers, you only notice when training looks off.

TLDR: warn when add_noise runs before timesteps are set.

just a warning + test, no behavior change. happy to send a PR if that sounds right.

version: diffusers main