[moe training] change api _scaled_grouped_mm -> _quantize_then_scaled_grouped_mm

[danielvegamyhre]

Renaming user facing API to be more explicit, to avoid potential confusion with torch._scaled_grouped_mm.

Test plan

• pytest test/prototype/moe_training/ -s

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/ao/3218

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✅ You can merge normally! (1 Unrelated Failure)

As of commit a1473c4 with merge base ca99c1c ():

BROKEN TRUNK - The following job failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• Run Regression Tests (aarch64) / test-cpu-ops (macos-14) (gh) (trunk failure)
  /Users/runner/miniconda3/envs/venv/lib/python3.10/site-packages/executorch/share/cmake/../../include/executorch/runtime/core/memory_allocator.h:143:21: error: no matching function for call to 'mul_overflows'

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