[feat]: General diffusers adapter backend to run diffusion models (#2724)

Signed-off-by: Huang, Zeyu <11222265+fhfuih@users.noreply.github.com>

tests/e2e/online_serving/test_diffusers_adapter.py

@@ -0,0 +1,56 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+
+"""End-to-end tests for DiffusersAdapterPipeline.
+
+It tests the full user flow of launching a diffusers-backed model and running inference.
+"""
+
+import pytest
+from PIL import Image
+
+from tests.helpers.mark import hardware_test
+from tests.helpers.runtime import OmniServer, OmniServerParams, OpenAIClientHandler, dummy_messages_from_mix_data
+
+pytestmark = [pytest.mark.diffusion, pytest.mark.core_model]
+
+
+@pytest.mark.parametrize(
+    "omni_server",
+    [
+        OmniServerParams(
+            model="tiny-random/Qwen-Image",
+            server_args=[
+                "--diffusion-load-format",
+                "diffusers",
+                "--diffusers-call-kwargs",
+                '{"height": 512, "width": 0}',  # deliberately weird width to be overridden
+            ],
+        ),
+    ],
+    indirect=True,
+)
+@hardware_test(res={"cuda": "L4"}, num_cards=1)
+def test_t2i_with_diffusers_adapter(
+    omni_server: OmniServer,
+    openai_client: OpenAIClientHandler,
+):
+    messages = dummy_messages_from_mix_data(content_text="a photo of an astronaut riding a horse on mars")
+
+    request_config = {
+        "model": omni_server.model,
+        "messages": messages,
+        "extra_body": {
+            "width": 512,
+            "num_inference_steps": 2,
+            "negative_prompt": "blurry",
+            "true_cfg_scale": 4.0,
+            "seed": 42,
+        },
+    }
+
+    response = openai_client.send_diffusion_request(request_config)
+    image: Image.Image = response[0].images[0]  # pyright: ignore[reportOptionalSubscript]
+
+    # Request config has incomplete width/height, so internal assertion in `send_diffusion_request` is incomplete.
+    assert image.size == (512, 512)