[llvm] [mlir] [MLIR] Add f6E3M2FN type (PR #105573)

[Diego Caballero via llvm-commits]

dcaballe wrote:

> Just curious as there have been various conversations: have we found a use case for the elemental form of f6 yet? When I've drilled in before, it seemed like there might be a case for that but all uses in the wild I could see were for the shared scale forms (which are definitely not a scalar type).

I believe the goal is to use the element types as a building block to model block-scaled types. After looking at how block-scaled types are modeled in other projects, I reached the conclusion that we need to build more expertise before introducing the full concept of a block-scaled type in MLIR, if at all. By using element types, we could experiment with block-scaled types by modeling them with separate tensors: one for all the block elements and another for all the block scales. Regardless of our final approach, we would need this basic support in `APFloat` to handle their specific semantics.

https://github.com/llvm/llvm-project/pull/105573