[chore] bring PipelineQuantizationConfig at the top of the import chain. (#11656)

bring PipelineQuantizationConfig at the top of the import chain.

src/diffusers/__init__.py

@@ -692,6 +692,7 @@ else:
 
 if TYPE_CHECKING or DIFFUSERS_SLOW_IMPORT:
     from .configuration_utils import ConfigMixin
+    from .quantizers import PipelineQuantizationConfig
 
     try:
         if not is_bitsandbytes_available():