[model-gateway] Implement Zero-Copy Vision Tensor Access (#15750)

sgl-model-gateway/src/multimodal/vision/image_processor.rs

@@ -3,7 +3,7 @@
 //! This module defines the interface for model-specific image processors
 //! and the common output format for preprocessed images.
 
-use std::collections::HashMap;
+use std::{borrow::Cow, collections::HashMap};
 
 use image::DynamicImage;
 use ndarray::{Array4, ArrayD};
@@ -206,9 +206,12 @@ impl PreprocessedImages {
         self.num_img_tokens.iter().sum()
     }
 
-    /// Get pixel values as a flat f32 slice (row-major order).
-    pub fn pixel_values_flat(&self) -> Vec<f32> {
-        self.pixel_values.iter().copied().collect()
+    /// Get pixel values as a flat f32 slice without copying if possible.
+    pub fn pixel_values_flat(&self) -> Cow<'_, [f32]> {
+        match self.pixel_values.as_slice() {
+            Some(slice) => Cow::Borrowed(slice),
+            None => Cow::Owned(self.pixel_values.iter().copied().collect()),
+        }
     }
 
     /// Get the shape of pixel values as a vector.