[Model][Gemma3] Simplify image input validation (#18710)

Signed-off-by: Lukas Geiger <lukas.geiger94@gmail.com>

vllm/model_executor/models/gemma3_mm.py

@@ -504,18 +504,12 @@ class Gemma3ForConditionalGeneration(nn.Module, SupportsMultiModal, SupportsPP,
         return next(self.parameters()).dtype
 
     def _validate_pixel_values(self, data: torch.Tensor) -> torch.Tensor:
-        h = w = self.config.vision_config.image_size
-        expected_dims = (3, h, w)
-
-        def _validate_shape(d: torch.Tensor):
-            if d.shape != expected_dims:
+        image_size = self.config.vision_config.image_size
+        expected_dims = (3, image_size, image_size)
+        if data.shape[1:] != expected_dims:
             raise ValueError(
-                    "The expected shape of pixel values per image per batch "
-                    f"is {expected_dims}. You supplied {tuple(d.shape)}.")
-
-        for d in data:
-            _validate_shape(d)
-
+                "The expected shape of pixel values per image per batch is "
+                f"{expected_dims}. You supplied {tuple(data.shape)}.")
         return data
 
     def _parse_and_validate_image_input(