[MLIR][NVGPU] Introduce `warpgroup.init.accumulator` Op (#67530)

This Op generates and initilizes the accumulator matrix for
`nvgpu.warpgroup.mma` op to perform matrix-multiply-and-accumulate
(mma).

Its associated transformation generates `!llvm.struct<>` and fill it
with the initial values. The size of struct is number of required inout
registers for `nvgpu.warpgroup.mma` op.