Support different value types in `cub::DeviceMerge`

`cub::DeviceMerge` does not officially (as documented) support merging sequences with different key or value types. Historically, `thrust::merge_by_key` before merging #1817 did support this in a limited fashion. We should dedicate some effort to make `cub::DeviceMerge` properly support mixing input types.

E.g., merging a key-value sequence of (int, short) with a sequence of (unsigned, long) into a sequence of (long, int). This requires us to avoid over- and underflow when comparing key types and possibly different register storage arrays for the input sequences, since a common key type between the two input key types may not be comparable by the comparison predicate.