[quant] allow `components_to_quantize` to be a non-list for single components (#12234)

* allow non list components_to_quantize.

* up

* Apply suggestions from code review

* Apply suggestions from code review
Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>

* [docs] components_to_quantize (#12287)

init
Co-authored-by: Sayak Paul <spsayakpaul@gmail.com>

---------
Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>

docs/source/en/api/pipelines/cogvideox.md

@@ -50,7 +50,7 @@ from diffusers.utils import export_to_video
 pipeline_quant_config = PipelineQuantizationConfig(
   quant_backend="torchao",
   quant_kwargs={"quant_type": "int8wo"},
-  components_to_quantize=["transformer"]
+  components_to_quantize="transformer"
 )
 
 # fp8 layerwise weight-casting

docs/source/en/api/pipelines/hunyuan_video.md

@@ -54,7 +54,7 @@ pipeline_quant_config = PipelineQuantizationConfig(
       "bnb_4bit_quant_type": "nf4",
       "bnb_4bit_compute_dtype": torch.bfloat16
       },
-    components_to_quantize=["transformer"]
+    components_to_quantize="transformer"
 )
 
 pipeline = HunyuanVideoPipeline.from_pretrained(
@@ -91,7 +91,7 @@ pipeline_quant_config = PipelineQuantizationConfig(
       "bnb_4bit_quant_type": "nf4",
       "bnb_4bit_compute_dtype": torch.bfloat16
       },
-    components_to_quantize=["transformer"]
+    components_to_quantize="transformer"
 )
 
 pipeline = HunyuanVideoPipeline.from_pretrained(
@@ -139,7 +139,7 @@ export_to_video(video, "output.mp4", fps=15)
         "bnb_4bit_quant_type": "nf4",
         "bnb_4bit_compute_dtype": torch.bfloat16
         },
-      components_to_quantize=["transformer"]
+      components_to_quantize="transformer"
   )
 
   pipeline = HunyuanVideoPipeline.from_pretrained(

docs/source/en/quantization/overview.md

@@ -34,7 +34,9 @@ Initialize [`~quantizers.PipelineQuantizationConfig`] with the following paramet
 > [!TIP]
 > These `quant_kwargs` arguments are different for each backend. Refer to the [Quantization API](../api/quantization) docs to view the arguments for each backend.
 
-- `components_to_quantize` specifies which components of the pipeline to quantize. Typically, you should quantize the most compute intensive components like the transformer. The text encoder is another component to consider quantizing if a pipeline has more than one such as [`FluxPipeline`]. The example below quantizes the T5 text encoder in [`FluxPipeline`] while keeping the CLIP model intact.
+- `components_to_quantize` specifies which component(s) of the pipeline to quantize. Typically, you should quantize the most compute intensive components like the transformer. The text encoder is another component to consider quantizing if a pipeline has more than one such as [`FluxPipeline`]. The example below quantizes the T5 text encoder in [`FluxPipeline`] while keeping the CLIP model intact.
+
+   `components_to_quantize` accepts either a list for multiple models or a string for a single model.
 
 The example below loads the bitsandbytes backend with the following arguments from [`~quantizers.quantization_config.BitsAndBytesConfig`], `load_in_4bit`, `bnb_4bit_quant_type`, and `bnb_4bit_compute_dtype`.
 
@@ -62,6 +64,7 @@ pipe = DiffusionPipeline.from_pretrained(
 image = pipe("photo of a cute dog").images[0]
 ```
 
+
 ### Advanced quantization
 
 The `quant_mapping` argument provides more options for how to quantize each individual component in a pipeline, like combining different quantization backends.

docs/source/en/using-diffusers/text-img2vid.md

@@ -98,7 +98,7 @@ pipeline_quant_config = PipelineQuantizationConfig(
     "bnb_4bit_quant_type": "nf4",
     "bnb_4bit_compute_dtype": torch.bfloat16
     },
-  components_to_quantize=["transformer"]
+  components_to_quantize="transformer"
 )
 
 pipeline = HunyuanVideoPipeline.from_pretrained(

src/diffusers/quantizers/pipe_quant_config.py

@@ -48,12 +48,15 @@ class PipelineQuantizationConfig:
         self,
         quant_backend: str = None,
         quant_kwargs: Dict[str, Union[str, float, int, dict]] = None,
-        components_to_quantize: Optional[List[str]] = None,
+        components_to_quantize: Optional[Union[List[str], str]] = None,
         quant_mapping: Dict[str, Union[DiffQuantConfigMixin, "TransformersQuantConfigMixin"]] = None,
     ):
         self.quant_backend = quant_backend
         # Initialize kwargs to be {} to set to the defaults.
         self.quant_kwargs = quant_kwargs or {}
+        if components_to_quantize:
+            if isinstance(components_to_quantize, str):
+                components_to_quantize = [components_to_quantize]
         self.components_to_quantize = components_to_quantize
         self.quant_mapping = quant_mapping
         self.config_mapping = {}  # book-keeping Example: `{module_name: quant_config}`

tests/quantization/test_pipeline_level_quantization.py

@@ -299,3 +299,19 @@ transformer BitsAndBytesConfig {
         data = json.loads(json_part)
 
         return data
+
+    def test_single_component_to_quantize(self):
+        component_to_quantize = "transformer"
+        quant_config = PipelineQuantizationConfig(
+            quant_backend="bitsandbytes_8bit",
+            quant_kwargs={"load_in_8bit": True},
+            components_to_quantize=component_to_quantize,
+        )
+        pipe = DiffusionPipeline.from_pretrained(
+            self.model_name,
+            quantization_config=quant_config,
+            torch_dtype=torch.bfloat16,
+        )
+        for name, component in pipe.components.items():
+            if name == component_to_quantize:
+                self.assertTrue(hasattr(component.config, "quantization_config"))