X8 version of Constand Pad ND operator

Generalize implementation of Constant Pad operator to 8-bit element types.

PiperOrigin-RevId: 389646484
diff --git a/src/xnnpack/operator.h b/src/xnnpack/operator.h
index 8a08e28..328dbaf 100644
--- a/src/xnnpack/operator.h
+++ b/src/xnnpack/operator.h
@@ -41,11 +41,12 @@
   xnn_operator_type_average_pooling_nhwc_f32,
   xnn_operator_type_average_pooling_nhwc_qu8,
   xnn_operator_type_bankers_rounding_nc_f32,
-  xnn_operator_type_channel_shuffle_nc_x32,
   xnn_operator_type_channel_shuffle_nc_x8,
+  xnn_operator_type_channel_shuffle_nc_x32,
   xnn_operator_type_clamp_nc_f32,
   xnn_operator_type_clamp_nc_u8,
   xnn_operator_type_ceiling_nc_f32,
+  xnn_operator_type_constant_pad_nd_x8,
   xnn_operator_type_constant_pad_nd_x32,
   xnn_operator_type_convolution_nchw_f32,
   xnn_operator_type_convolution_nhwc_f16,