auto type conversion (#4270)

* type conversion

Default value of `control_guidance_start` and `control_guidance_end` in `StableDiffusionControlNetPipeline.check_inputs` causes `TypeError: object of type 'float' has no len()`

Proposed fix: 
Convert `control_guidance_start` and `control_guidance_end` to list if float

* Update src/diffusers/pipelines/controlnet/pipeline_controlnet.py

* Update src/diffusers/pipelines/controlnet/pipeline_controlnet.py
Co-authored-by: Patrick von Platen <patrick.v.platen@gmail.com>

* Update src/diffusers/pipelines/controlnet/pipeline_controlnet.py
Co-authored-by: Patrick von Platen <patrick.v.platen@gmail.com>

---------
Co-authored-by: Patrick von Platen <patrick.v.platen@gmail.com>

src/diffusers/pipelines/controlnet/pipeline_controlnet.py

@@ -561,6 +561,12 @@ class StableDiffusionControlNetPipeline(
         else:
             assert False
 
+        if not isinstance(control_guidance_start, (tuple, list)):
+            control_guidance_start = [control_guidance_start]
+            
+        if not isinstance(control_guidance_end, (tuple, list)):
+            control_guidance_end = [control_guidance_end]
+            
         if len(control_guidance_start) != len(control_guidance_end):
             raise ValueError(
                 f"`control_guidance_start` has {len(control_guidance_start)} elements, but `control_guidance_end` has {len(control_guidance_end)} elements. Make sure to provide the same number of elements to each list."