Add test for solver_type

This test currently fails in main. When switching from DEIS to UniPC,
solver_type is "logrho" (the default value from DEIS), which gets
translated to "bh1" by UniPC. This is different to the default value for
UniPC: "bh2". This is where the translation happens: 36d22d0709/src/diffusers/schedulers/scheduling_unipc_multistep.py (L171)

tests/schedulers/test_schedulers.py

@@ -25,11 +25,13 @@ import torch
 import diffusers
 from diffusers import (
     DDIMScheduler,
+    DEISMultistepScheduler,
     DiffusionPipeline,
     EulerAncestralDiscreteScheduler,
     EulerDiscreteScheduler,
     IPNDMScheduler,
     LMSDiscreteScheduler,
+    UniPCMultistepScheduler,
     VQDiffusionScheduler,
     logging,
 )
@@ -229,6 +231,19 @@ class SchedulerBaseTests(unittest.TestCase):
         pipe.scheduler = LMSDiscreteScheduler.from_config(pipe.scheduler.config)
         assert pipe.scheduler.config.timestep_spacing == "trailing"
 
+    def test_default_solver_type_after_switch(self):
+        pipe = DiffusionPipeline.from_pretrained(
+            "hf-internal-testing/tiny-stable-diffusion-pipe", torch_dtype=torch.float16
+        )
+        assert pipe.scheduler.__class__ == DDIMScheduler
+
+        pipe.scheduler = DEISMultistepScheduler.from_config(pipe.scheduler.config)
+        assert pipe.scheduler.config.solver_type == "logrho"
+
+        # Switch to UniPC, verify the solver is the default
+        pipe.scheduler = UniPCMultistepScheduler.from_config(pipe.scheduler.config)
+        assert pipe.scheduler.config.solver_type == "bh2"
+
 
 class SchedulerCommonTest(unittest.TestCase):
     scheduler_classes = ()