[DPMSolverSinglestepScheduler] correct get_order_list for solver_order=2and lower_order_final=True

[yiyixuxu]

For lower_order_final=True and solver_order=2, we would expect the order for the last step to be 1, currently inDPMSolverSinglestepScheduler it does not work as expected for an even number of steps

this is the get_order_list method of DPMSolverSinglestepScheduler, (see my comments in code below)

diffusers/src/diffusers/schedulers/scheduling_dpmsolver_singlestep.py

Line 216 in 0a1daad

def get_order_list(self, num_inference_steps: int) -> List[int]:

currently lower_order_final=True and lower_order_final=False have exactly same result

    def get_order_list(self, num_inference_steps: int) -> List[int]:
        steps = num_inference_steps
        order = self.config.solver_order
        if self.config.lower_order_final:
            if order == 3:
                ...
            elif order == 2:
                if steps % 2 == 0:
                    # YiYi notes: for solver_order = 2, if the steps are even, it seems that we are not lowering the final order even when `config.lower_final_order=True`
                    orders = [1, 2] * (steps // 2) 
                else:
                    # YiYi notes: for uneven steps however, it lowers the final order as expected
                    orders = [1, 2] * (steps // 2) + [1]
            elif order == 1:
              ...
        else:
           ...
           elif order == 2:
              # YiYi notes: this is exactly same as lower_final_order=True
              orders = [1, 2] * (steps // 2)
           ...
        return orders

another option is just to not allow use even number of steps + solver_orde=2 + lower_final_order - I think it's more complicated that way, especially now we have the `last_sigmas_type='zero' argument that has to work with final_order ==1

related PR /issue
#6949
#1866
#3413
#6477

[yiyixuxu]

I think it's a typo here but let me know if it's not @patrickvonplaten
from this PR #3413

[patrickvonplaten]

Yes LGTM - thanks!

[yiyixuxu]

for order=2 and lower_order_final=True, currently it generate orders like 1,2,1,2...,1,2 so final order will be 2
changing it to have both of the last two final steps to be 1 here

[patrickvonplaten]

Is this only necessary for DPMSingleStepSolver? Not for the other ones?

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[patrickvonplaten]

This looks good to me! Can we quickly make sure that other schedulers (like multi-step, unipc and/or deis) are not affected by this bug?

[yiyixuxu]

@patrickvonplaten

This looks good to me! Can we quickly make sure that other schedulers (like multi-step, unipc and/or deis) are not affected by this bug?

I think this is unique to singlestep

[yiyixuxu]

I changed the default value so that we keep the default behavior the same as before @patrickvonplaten
this way I don't have to update test