[Update] Add benchmark and autotune for group_gemm

[xjmxyt]

Description

Previously, we don't provide autotuner for group_gemm and also benchmarks. Now add one.

CI Configuration

config:
  build: true
  # valid options are "ops" and "benchmark"
  test: ["ops", "benchmark"]

Checklist

• Code formatted and imports sorted via repo specifications (./format.sh)
• Documentation updated (if needed)
• CI configuration reviewed

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[xjmxyt]

/ok to test f67f4e3

[xjmxyt]

/ok to test 69d3bbd

[hannahli-nv]

LGTM, thx.