[mlir][nvgpu] Improve nvgpu->nvvm transformation of `warpgroup.mma` Op (NFC) (#67325)

This PR introduces substantial improvements to the readability and
maintainability of the `nvgpu.warpgroup.mma` Op transformation from
nvgpu->nvvm. This transformation plays a crucial role in GEMM and
manages complex operations such as generating multiple wgmma ops and
iterating their descriptors. The prior code lacked clarity, but this PR
addresses that issue effectively.

**PR does followings:**
**Introduces a helper class:** `WarpgroupGemm` class encapsulates the
necessary functionality, making the code cleaner and more
understandable.

**Detailed Documentation:** Each function within the helper class is
thoroughly documented to provide clear insights into its purpose and
functionality.