[FEA] chunked_pack should allow variable sized spans in the next call

[abellina]

As brought up here #13260 (comment), chunked_pack requires that it be configured with a single size for the user's bounce buffer, and further requires the same size of device memory made available in every call to chunked_pack::next.

This is overkill in the case where we have actually very little work to be done (the source table is small). So the proposal is to allow for a get_next_size() api that lets us know the size of buffer that should be made available to next

[abellina]

Note that there was more feedback #13260 (review) that it would be good to tackle as a clean up PR separate from this particular issue.