Support ragged inputs to our text generation utilites

[mattdangerw]

For the text generation utilities listed here, it would be nice to support ragged inputs, so we could remove the constraint that all seed text in a given batch needs to be exactly the same way.

The simplest way to do this would be to also support a ragged prompt for the generation utils.

[abheesht17]

Will be glad to take this up!

[mattdangerw]

Thanks!