[DT] Make the padding sizes of encodings be a scalar.

[lialan]

• rename round_dims_to to max_padding
• make max_padding a generic case single value attribute

Progress towards https://github.com/iree-org/iree/issues/17553

[lialan]

I think a better solution could be using std::optional<int64_t> padFactor. It's more aligned with how we define the padFactor in EncodingBased.td.

@hanhanW I it is much better to do in this way. I will do a subsequent PR for it later. For now just leave it as is.

[hanhanW]

@lialan I took a stab at better PR title and linked the related issue to the PR description. Feel free to make it better if you got some ideas. (I'm not good at naming.)

[lialan]

Dropping this in favor of https://github.com/iree-org/iree/issues/17729