Write the generated images using the image crate. (#363)

* Use the image crate to write the generated images.

* Make the dependency optional.

candle-examples/examples/stable-diffusion/utils.rs

@@ -10,3 +10,22 @@ pub fn linspace(start: f64, stop: f64, steps: usize) -> Result<Tensor> {
         .collect::<Vec<_>>();
     Tensor::from_vec(vs, steps, &Device::Cpu)
 }
+
+/// Saves an image to disk using the image crate, this expects an input with shape
+/// (c, width, height).
+pub fn save_image<P: AsRef<std::path::Path>>(img: &Tensor, p: P) -> Result<()> {
+    let p = p.as_ref();
+    let (channel, width, height) = img.dims3()?;
+    if channel != 3 {
+        candle::bail!("save_image expects an input of shape (3, width, height)")
+    }
+    let img = img.transpose(0, 1)?.t()?.flatten_all()?;
+    let pixels = img.to_vec1::<u8>()?;
+    let image: image::ImageBuffer<image::Rgb<u8>, Vec<u8>> =
+        match image::ImageBuffer::from_raw(width as u32, height as u32, pixels) {
+            Some(image) => image,
+            None => candle::bail!("error saving image {p:?}"),
+        };
+    image.save(p).map_err(candle::Error::wrap)?;
+    Ok(())
+}