implement of leftpadding

[pass-lin]

Left padding is now the mainstream LLM padding method. However, keras_hub does not have a corresponding implementation.

Does the keras-team have a corresponding implementation plan? Or can I offer my help and implement it myself?

If it were me to implement this, I would add a "use_left_padding" parameter on top of StartEndPacker.

[mattdangerw]

I think padding_side="left" and padding_side="right" would probably be a better name (keep right padding as the default)?

I would add this to StartEndPacker and MultiSegmentPacker in one go. The trickier part would be to support this in generation, that would need some bigger updates. But I don't see an issue with supporting this in our layers for a start.

[mattdangerw]

@pass-lin feel free to open a PR if above makes sense to you!