Request the UniPCMultistepScheduler to allow a customed timesteps

**Is your feature request related to a problem? Please describe.**
Current UniPC Solver don't allow user set the timesteps by himself. Although it allow user set the sigmas, but that is for those diffusion model who allow the use_flow_sigmas to be true.

**Describe the solution you'd like.**
provide a set_timesteps function similar to the DPM Multistep Solver, which allow user set the timesteps by himself.

**Describe alternatives you've considered.**
I do think allowing users to set sigma even when the use_flow_sigmas is false is also an option, but that is not align with the previous solution in the DPM Multistep Solver

**Additional context.**
ODE solver for SD is still a popular research area, lots of algorithm, like LD3 from Nvidia, search a good timeschedule for the LDM.

Inizia individuando UniPCMultistepScheduler e confronta la gestione dei suoi timestep con la funzione set_timesteps di DPM Multistep Solver. Verifica come i timestep personalizzati interagiscono quando use_flow_sigmas è false; il lavoro è completato quando gli utenti possono fornire i propri timestep tramite lo scheduler e il comportamento è coperto dai test pertinenti dello scheduler.

Scritto dal modello di indicizzazione a partire dal testo della issue.