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/operator-strings.c b/src/operator-strings.c
index 4f706d4..8f92ce1 100644
--- a/src/operator-strings.c
+++ b/src/operator-strings.c
@@ -38,14 +38,16 @@
       return "Bankers Rounding (NC, F32)";
     case xnn_operator_type_ceiling_nc_f32:
       return "Ceiling (NC, F32)";
-    case xnn_operator_type_channel_shuffle_nc_x32:
-      return "Channel Shuffle (NC, X32)";
     case xnn_operator_type_channel_shuffle_nc_x8:
       return "Channel Shuffle (NC, X8)";
+    case xnn_operator_type_channel_shuffle_nc_x32:
+      return "Channel Shuffle (NC, X32)";
     case xnn_operator_type_clamp_nc_f32:
       return "Clamp (NC, F32)";
     case xnn_operator_type_clamp_nc_u8:
       return "Clamp (NC, U8)";
+    case xnn_operator_type_constant_pad_nd_x8:
+      return "Constant Pad (ND, X8)";
     case xnn_operator_type_constant_pad_nd_x32:
       return "Constant Pad (ND, X32)";
     case xnn_operator_type_convolution_nhwc_f16: