Reject an empty custom timestep list with a clear ValueError

[alex-16moro]

Why

DDPMScheduler.set_timesteps(timesteps=[]) currently fails with IndexError: list index out of range when it reads timesteps[0], which gives the caller no hint about what went wrong.

What changed

• DDPMScheduler.set_timesteps raises ValueError ("timesteps cannot be empty.") for an empty custom list. The check runs after the existing mutual-exclusion check and before timesteps[0] is read.
• DDPMParallelScheduler.set_timesteps is a # Copied from copy and was updated with utils/check_copies.py --fix_and_overwrite.
• Both docstrings now state that the list must be non-empty.
• Valid lists, single-element lists and num_inference_steps schedules behave as before.

Considered and rejected: substituting a default such as [999]. The library's guidance prefers a concise error to silently correcting input, and 999 is only meaningful when num_train_timesteps is 1000.

Validation

Ramp Kit full verification: 12/12 checks PASS (acceptance, scheduler tests, lint, format, upstream quality, copies, dummies, support list, forward docstrings, dependency table, dependencies, test strength). The test-strength replay removed the fix and the new test failed as intended. Patch digest 02c34f9999a89c1131050dd001c3e7a0bf1dbb7b4f18db93b0b4bf62108ad3da.

Reviewer attention

• Error wording, and precedence: when both arguments are passed, the existing mutual-exclusion error still wins.
• The parallel-scheduler test is in tests/schedulers/test_scheduler_ddpm.py because the kit's editable scope excludes the parallel test file; it can move if preferred.