async function getImage() {
  const img = await webcam.capture();
  const processedImg =
      tf.tidy(() => img.expandDims(0).toFloat().div(127).sub(1));
  img.dispose();
  return processedImg;
}
Captures a frame from the webcam and normalizes it between -1 and 1.
Returns a batched image (1-element batch) of shape [1, w, h, c].