Add Auto Sizes to Lazy Loaded images

add_filter( 'wp_get_attachment_image_attributes', function( $attr ) {

	if ( ! isset( $attr['loading'] ) || 'lazy' !== $attr['loading'] || ! isset( $attr['sizes'] ) ) {

		return $attr;

	}

	// Skip if attribute was already added.

	if ( false !== strpos( $attr['sizes'], 'auto,' ) ) {

		return $attr;

	}

	$attr['sizes'] = 'auto, ' . $attr['sizes'];

	return $attr;

} );

add_filter( 'wp_content_img_tag',  function( $html ) {

	if (false === strpos($html, 'loading="lazy"') || (false === strpos($html, 'sizes="') || false !== strpos($html, 'sizes="auto,'))) {

		return $html;

	}

	$html = str_replace( 'sizes="', 'sizes="auto, ', $html );

	return $html;

} );