[MLIR][NVVM] Explicit Data Type for Output in `wgmma.mma_async` (#78713)

The current implementation of `nvvm.wgmma.mma_async` Op deduces the data
type of the output matrix from the data type of struct member, which can be
non-intuitive, especially in cases where types like `2xf16` are packed
into `i32`.

This PR addresses this issue by improving the Op to include an explicit
data type for the output matrix.

The modified Op now includes an explicit data type for Matrix-D (<f16>),
and looks as follows:

```
%result = llvm.mlir.undef : !llvm.struct<(struct<(i32, i32, ...
nvvm.wgmma.mma_async
    %descA, %descB, %result,
    #nvvm.shape<m = 64, n = 32, k = 16>,
    D [<f16>, #nvvm.wgmma_scale_out<zero>],
    A [<f16>, #nvvm.wgmma_scale_in<neg>, <col>],
    B [<f16>, #nvvm.wgmma_scale_in<neg>, <col>]
```