title: Add Quantity Input Field on WooCommerce Checkout For Each Product
body: <p>I am trying to understand how I can add the quantity input field on the checkout page and that way, allow the customer to change the quantity for each product.</p>
  
  <p><strong>This is what I got so far:</strong></p>
  
  <pre><code>add_filter( 'woocommerce_checkout_cart_item_quantity', 'qty_and_qty_change_on_checkout', 20, 3 );
  function qty_and_qty_change_on_checkout( $quantity_html, $cart_item, $cart_item_key ) {
  
      return '&lt;br&gt;
      '.woocommerce_quantity_input().'
      &lt;span class="product-quantity"&gt;
      ' . sprintf( '&lt;b&gt;Qty:&lt;/b&gt; %s', $cart_item['quantity'] ) . '
      &lt;/span&gt;';
  }
  </code></pre>
  
  <p>But this gives me an error even though <code>woocommerce_quantity_input()</code> is the official function for it? <strong>The error is:</strong></p>
  
  <pre><code>Notice: Undefined index: product in /wp-content/plugins/woocommerce/includes/wc-template-functions.php on line 1670
  </code></pre>