[C++] Make `float16` part of `FloatingPointTypes()`

[fornwall]

Describe the enhancement requested

FloatingPointTypes() currently contains only float32() and float64(), so float16 is also absent from NumericTypes() and PrimitiveTypes().

As a result, every kernel registration loop and test suite that iterates over those lists skips float16, and support for it has to be added type by type instead. Several kernel families explicitly reject HalfFloatType with Status::NotImplemented today.

float16 should probably be a member of FloatingPointTypes(), consistent with the other floating point types.

Component(s)

C++

[VedantRalekar]

Hi, i would like to work on this issue, plese assign it to me.