[Misc] Build canonical image task prompts (#6049)

Signed-off-by: Hongsheng Liu <liuhongsheng4@huawei.com>

tests/examples/offline_inference/test_image_task_prompts.py

@@ -0,0 +1,47 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+
+from __future__ import annotations
+
+import pytest
+from PIL import Image
+
+from examples.offline_inference.image_to_video.image_to_video import build_image_to_video_prompt
+from examples.offline_inference.text_to_image.text_to_image import build_text_to_image_prompt
+
+pytestmark = [pytest.mark.core_model, pytest.mark.cpu]
+
+
+@pytest.mark.parametrize(
+    ("negative_prompt", "expected_negative_prompt"),
+    [(None, None), ("", ""), ("blurry", "blurry")],
+)
+def test_text_to_image_builds_canonical_prompt(
+    negative_prompt: str | None,
+    expected_negative_prompt: str | None,
+) -> None:
+    result = build_text_to_image_prompt("a red fox", negative_prompt)
+
+    assert result["prompt"] == "a red fox"
+    assert result["modalities"] == ["image"]
+    if expected_negative_prompt is None:
+        assert "negative_prompt" not in result
+    else:
+        assert result["negative_prompt"] == expected_negative_prompt
+
+
+def test_image_to_video_builds_canonical_prompt() -> None:
+    image = Image.new("RGB", (32, 16), "red")
+
+    result = build_image_to_video_prompt(
+        prompt="the fox turns toward the camera",
+        negative_prompt="flicker",
+        media_inputs={"image": image},
+    )
+
+    assert result == {
+        "prompt": "the fox turns toward the camera",
+        "modalities": ["video"],
+        "multi_modal_data": {"image": image},
+        "negative_prompt": "flicker",
+    }