Fix DPM single (#3413)

* Fix DPM single

* add test

* fix one more bug

* Apply suggestions from code review
Co-authored-by: StAlKeR7779 <stalkek7779@yandex.ru>

---------
Co-authored-by: StAlKeR7779 <stalkek7779@yandex.ru>

src/diffusers/schedulers/scheduling_dpmsolver_singlestep.py

@@ -21,9 +21,13 @@ import numpy as np
 import torch
 
 from ..configuration_utils import ConfigMixin, register_to_config
+from ..utils import logging
 from .scheduling_utils import KarrasDiffusionSchedulers, SchedulerMixin, SchedulerOutput
 
 
+logger = logging.get_logger(__name__)  # pylint: disable=invalid-name
+
+
 # Copied from diffusers.schedulers.scheduling_ddpm.betas_for_alpha_bar
 def betas_for_alpha_bar(num_diffusion_timesteps, max_beta=0.999):
     """
@@ -251,7 +255,14 @@ class DPMSolverSinglestepScheduler(SchedulerMixin, ConfigMixin):
         self.timesteps = torch.from_numpy(timesteps).to(device)
         self.model_outputs = [None] * self.config.solver_order
         self.sample = None
-        self.orders = self.get_order_list(num_inference_steps)
+
+        if not self.config.lower_order_final and num_inference_steps % self.config.solver_order != 0:
+            logger.warn(
+                "Changing scheduler {self.config} to have `lower_order_final` set to True to handle uneven amount of inference steps. Please make sure to always use an even number of `num_inference steps when using `lower_order_final=True`."
+            )
+            self.register_to_config(lower_order_final=True)
+
+        self.order_list = self.get_order_list(num_inference_steps)
 
     # Copied from diffusers.schedulers.scheduling_ddpm.DDPMScheduler._threshold_sample
     def _threshold_sample(self, sample: torch.FloatTensor) -> torch.FloatTensor:
@@ -597,6 +608,12 @@ class DPMSolverSinglestepScheduler(SchedulerMixin, ConfigMixin):
         self.model_outputs[-1] = model_output
 
         order = self.order_list[step_index]
+
+        #  For img2img denoising might start with order>1 which is not possible
+        #  In this case make sure that the first two steps are both order=1
+        while self.model_outputs[-order] is None:
+            order -= 1
+
         # For single-step solvers, we use the initial value at each time with order = 1.
         if order == 1:
             self.sample = sample

tests/schedulers/test_scheduler_dpm_single.py

@@ -116,6 +116,22 @@ class DPMSolverSinglestepSchedulerTest(SchedulerCommonTest):
 
         return sample
 
+    def test_full_uneven_loop(self):
+        scheduler = DPMSolverSinglestepScheduler(**self.get_scheduler_config())
+        num_inference_steps = 50
+        model = self.dummy_model()
+        sample = self.dummy_sample_deter
+        scheduler.set_timesteps(num_inference_steps)
+
+        # make sure that the first t is uneven
+        for i, t in enumerate(scheduler.timesteps[3:]):
+            residual = model(sample, t)
+            sample = scheduler.step(residual, t, sample).prev_sample
+
+        result_mean = torch.mean(torch.abs(sample))
+
+        assert abs(result_mean.item() - 0.2574) < 1e-3
+
     def test_timesteps(self):
         for timesteps in [25, 50, 100, 999, 1000]:
             self.check_over_configs(num_train_timesteps=timesteps)