Make `thrust::discard_iterator`'s `value_type` `void`

[bernhardmgruber]

thrust::discard_iterator's value_type is any_assign, and not void. However, this is necessary to allow replacing cub::DiscardOutputIterator by thrust::discard_iterator.

[bernhardmgruber]

I am not sure what to do about this. Pure output iterators in the standard, like std::ostream_iterator and std::back_insert_iterator have a void value_type, but that does not play well with zip_iterator. There is a hack in place, IIRC, to make CUB recognize thrust::discard_iterator, but I think it's desireable to fix zip_iterator allow iterators with value_type void. The problem is that zip_iterator forms a tuple<It::value_type...> and we cannot have a tuple<..., void, ...>. A workaround may be that if we detect any iterator to have a value_type of void to also make the entire zip_iterator's value_type void.

[bernhardmgruber]

This is solved by #4717 introducing cuda::discard_iterator.