Bugfix for PixtralHF models without spatial_merge_size (#16513)

Signed-off-by: mgoin <mgoin64@gmail.com>

vllm/model_executor/models/pixtral.py

@@ -926,8 +926,9 @@ class PixtralHFEncoderInfo(VisionEncoderInfo[PixtralVisionConfig]):
         return self.vision_config.image_size
 
     def get_patch_size(self) -> int:
-        return (self.vision_config.patch_size *
-                self.vision_config.spatial_merge_size)
+        spatial_merge_size = getattr(self.vision_config, "spatial_merge_size",
+                                     1)
+        return (self.vision_config.patch_size * spatial_merge_size)
 
     def get_patch_grid_length(self) -> int:
         image_size, patch_size = self.get_image_size(), self.get_patch_size()