Allow to enable half and bfloat16 at the same time

[yhmtsai]

This PR allow to enable bfloat16 and half precision at the same time.
Doing the operation between bfloat16 and half precision returns float type.
It will revert the gko::float16 alias changes in #1825 . float16 will be half precision again.
Moreover, it adds the next_precision_move<type, move> to represent the next_precision<next_precision<.... same for previous_precision_move

likely in another PR:

• convert_to are just copy the same implementation for different interface. create a templated convert_to in private: and convert_to always call the templated function
• instantiation macro, which we might apply the same technique as what I did in batch to use nested structure
• allow run<> to accepts type_list and create a list for that

[MarcelKoch]

Some first notes, I still have to finish the rest of the PR. However, I think these are already the most relevant comments.

[MarcelKoch]

Looks mostly good. My comment on the ConvertibleTo stuff can be ignored for this PR.

[ginkgo-bot]

Error: PR already merged!