[llvm] [mlir] [mlir][mesh] Add all-scatter operation (PR #81218)

[Boian Petkantchin via llvm-commits]

sogartar wrote:

I am not sure what is the best name for this operation. Scatter is the inverse of gather. It makes sense that the inverse of all-gather would be all-scatter. But then all-scatter implies that all devices scatter their tensor, when we are actually slicing.
`Local split` is anther possible name.
If anyone has other suggestions I can change the name.

https://github.com/llvm/llvm-project/pull/81218