[Bugfix] Fix Qwen-Image min-size normalization for tiny requests (#2637)

Signed-off-by: David Chen <530634352@qq.com>

vllm_omni/diffusion/utils/size_utils.py

@@ -0,0 +1,20 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+
+"""Shared size normalization helpers for diffusion pipelines."""
+
+
+def normalize_min_aligned_size(height: int, width: int, alignment: int) -> tuple[int, int]:
+    """Clamp dimensions to the minimum valid aligned size.
+
+    This preserves floor-to-alignment behavior for normal requests while
+    preventing very small dimensions from collapsing to zero after alignment.
+    """
+
+    alignment = int(alignment)
+    if alignment <= 0:
+        raise ValueError(f"Expected positive alignment, got {alignment}")
+
+    normalized_height = max(alignment, (int(height) // alignment) * alignment)
+    normalized_width = max(alignment, (int(width) // alignment) * alignment)
+    return normalized_height, normalized_width