model_index: plain _class_name string, select the shipped pipeline with custom_pipeline

Hub tooling requires _class_name to be a string, while the [file, class] form diffusers accepts for
custom pipelines makes it print a configuration warning. A string plus custom_pipeline="pipeline"
resolves the same class (verified against the Hub) without the warning.

README.md

@@ -77,8 +77,8 @@ Every image below is generated by this pipeline with 30 steps at 1024 px.
 import torch
 from diffusers import DiffusionPipeline
 
-pipe = DiffusionPipeline.from_pretrained("AiArtLab/zen-image-edit", trust_remote_code=True,
-                                         dtype=torch.float16)
+pipe = DiffusionPipeline.from_pretrained("AiArtLab/zen-image-edit", custom_pipeline="pipeline",
+                                         trust_remote_code=True, dtype=torch.float16)
 pipe.enable_model_cpu_offload()   # 14.5 GB DiT + fp32 VAE decoder do not co-reside on 32 GB
 
 # text-to-image
@@ -94,8 +94,10 @@ image = pipe(prompt="Replace the woman in <image2> with the woman from <image1>;
              generator=torch.Generator("cuda").manual_seed(1234)).images[0]
 ```
 
-`trust_remote_code=True` pulls `pipeline.py` and `transformer.py` from this repo and runs them, so
-no clone is needed. Cloning works too and gives the class directly:
+`custom_pipeline="pipeline"` builds the shipped `pipeline.py` and `trust_remote_code=True` lets it run,
+so no clone is needed. (`_class_name` is kept a plain string in `model_index.json` because that is what
+Hub tooling expects; the `[file, class]` form diffusers also accepts makes the Hub print a
+configuration warning.) Cloning works too and gives the class directly:
 
 ```python
 from pipeline import ZenImageEditPipeline

model_index.json

@@ -1,8 +1,5 @@
 {
-  "_class_name": [
-    "pipeline",
-    "ZenImageEditPipeline"
-  ],
+  "_class_name": "ZenImageEditPipeline",
   "_diffusers_version": "0.41.0.dev0",
   "processor": [
     "transformers",
@@ -24,4 +21,4 @@
     "diffusers",
     "AutoencoderKLQwenImage21"
   ]
-}
+}