Implementation of DPMSolverSinglestepScheduler

[wonkyoc]

diffusers/src/diffusers/schedulers/scheduling_dpmsolver_singlestep.py

Lines 637 to 641 in a51b6cc

	m0, m1 = model_output_list[-1], model_output_list[-2]
	h, h_0 = lambda_t - lambda_s1, lambda_s0 - lambda_s1
	r0 = h_0 / h
	D0, D1 = m1, (1.0 / r0) * (m0 - m1)

AFAIK, this implementation is multistep. The first order of the scheduler is fine but the second/third orders are not correctly implemented. Am I missing something?

[sayakpaul]

cc: @yiyixuxu