message: Add int16 support for `tf.as_string`
  In `tf.as_string`, integers are mostly supported (`int8`, `int32`, `int64`)
  but not `int16`. This fix adds the `int16` support for `tf.as_string`.
diff: @@ -73,6 +73,7 @@ class AsStringOp : public OpKernel {
  }
  switch (dtype) {
  case DT_INT8:
  + case DT_INT16:
  case DT_INT32:
  strings::Appendf(&format_, "d");
  break;
  @@ -129,6 +130,7 @@ class AsStringOp : public OpKernel {
  ENCODE_TYPE(DT_FLOAT, float, format_);
  ENCODE_TYPE(DT_DOUBLE, double, format_);
  ENCODE_TYPE(DT_INT8, int8, format_);
  + ENCODE_TYPE(DT_INT16, int16, format_);
  case (DT_BOOL): {
  const auto& input_flat = input_tensor->flat<bool>();
  for (int i = 0; i < input_flat.size(); ++i) {
  