message: add the padding order description
diff: @@ -163,7 +163,10 @@ OPERATOR_SCHEMA(Pad)
  .NumInputs(1)
  .NumOutputs(1)
  .Attr("paddings",
  - "List of integers indicate the padding sizes, paddings's length should be the double of input's dimension.",
  + "List of integers indicate the padding sizes, paddings's length"
  + " should be the double of input's dimension. "
  + "The order should be axis_0_begin, axis_0_end, axis_1_begin, ...,"
  + " axis_n_begin, axis_n_end, n is input's dimension.",
  AttrType::INTS,
  true)
  .Attr("mode",
  