kernel: use tensor cores for flashinfer gqa kernels

[yzh119]

This pr fixes the kernel dispatch rule for flashinfer backend.

Motivation

When group size > 4, using tensor cores is faster.

Checklist

• Format your code according to the Contributor Guide.
• Add unit tests as outlined in the Contributor Guide.
• Update documentation as needed, including docstrings or example tutorials.

[zhyncs]

Amazing work!