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.

Start by locating UniPCMultistepScheduler and compare its timestep handling with the DPM Multistep Solver's set_timesteps function. Check how custom timesteps interact with use_flow_sigmas being false; done means users can supply their own timesteps through the scheduler with behavior covered by the relevant scheduler tests.

Written by the indexing model from the issue text.