[SYCL][Docs] Add std::hash and std::numeric_limits specialization for bfloat16

[steffenlarsen]

This commit adds the missing std::hash and std::numeric_limits specializations for the sycl::ext::oneapi::bfloat16 class.

[steffenlarsen]

@gmlueck - These are also specialized for sycl::half despite not being explicitly mentioned by the specification. Is it fine to add these without changing the extensions for bfloat16 or do we need associated extension spec mentions?

[gmlueck]

@gmlueck - These are also specialized for sycl::half despite not being explicitly mentioned by the specification. Is it fine to add these without changing the extensions for bfloat16 or do we need associated extension spec mentions?

I think we should update the bfloat16 extension spec to say that the extension does provide these specializations. Using sycl::half as a precedent is not a good idea because it is so poorly specified currently. (There is a SYCL spec issue about this.)

For example, we could add a new section to the spec titled something like "Standard C++ library specializations"

[aelovikov-intel]

Do we have any guarantees on the layout/size? If so, can we use bit_cast instead of friendship?

[steffenlarsen]

We do, but bit_cast in our implementation is only sometimes constexpr, so the friendship for numeric_limits lets it be unconditionally constexpr. For std::hash it should be fine to drop the friendship though!

[aelovikov-intel]

Subjective, but I prefer

// in global ns
template <> struct std::{type}<types...> { ... };

[steffenlarsen]

I don't have a strong preference, but the current style is what we use for hash in other headers, so I would like to keep it as is. Then we can change it at a global level if we want.