nt and latter discrete timestep in the diffusion chain.   prev_timestep (int) —
The previous discrete timestep in the diffusion chain.   sample (torch.FloatTensor) —
A current instance of a sample created by the diffusion process.   Returns 
torch.FloatTensor
  
The sample tensor at the previous timestep.
  One step for the second-order singlestep DPMSolver that computes the solution at time prev_timestep from the
time timestep_list[-2].  singl