[executorch][cuda] fuse gate/up MLP projections

[Gasoonjia]

Summary:
Fuse each gemma4_31b MLP's gate_proj|up_proj into a single [2*intermediate, hidden] coalesced-int4 matmul, applied by default in the CUDA export. This issues one activation-quant + one W4A8 matvec per layer instead of two, cutting per-token launch + activation-quant overhead in the launch-bound decode path. Only Q4_K (CudaCoalescedInt4Tensor) gate/up pairs are fused; any other quant type (e.g. Q6_K) is left as two matmuls (guarded, still correct).

decode length	main branch	current branch
512	42.2	44.80
2K	40.8	43.20
8K	40.0	42.23
32K	39.4	41.64
127K	35.5	38.41

Next Step: we will upsteam this kind of operator fusion into gemma4-31b model level when loading gguf. #20481 is the draft PR

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❌ 4 New Failures, 3 Unrelated Failures, 5 Unclassified Failures

As of commit 1c371e2 with merge base 1b726b2 ():

NEW FAILURES - The following jobs have failed:

• pull / android / run-emulator (gh)
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• pull / test-arm-backend-no-driver (test_pytest_ops_tosa) / linux-job (gh)
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• pull / unittest / linux / linux-job (gh)
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• pull / unittest-editable / linux / linux-job (gh)
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UNCLASSIFIED FAILURES - DrCI could not classify the following jobs because the workflow did not run on the merge base. The failures may be pre-existing on trunk or introduced by this PR:

• Build Aarch64 Linux Wheels / pytorch/executorch / build-wheel-py3_10-cpu-aarch64 (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  /__w/executorch/executorch/pytorch/executorch/backends/apple/coreml/runtime/inmemoryfs/inmemory_filesystem.cpp:722:48: error: ‘inmemoryfs::InMemoryFileSystem::InMemoryNode::Kind’ has not been declared
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• MLX / test-mlx / test-mlx (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
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• pull / test-arm-backend-public-api-backward-compatibility / linux-job (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  RuntimeError: Command docker exec -t cb31ad38d6fbdd5bd4952956e5c1aaca1d5f36835ed783ca671429bec78a0a8c /exec failed with exit code 127
• Test Metal Backend / test-model-metal-e2e (mistralai, Voxtral-Mini-4B-Realtime-2602, quantized-int4-metal) / macos-job (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

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

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

• pull / test-llama-runner-qnn-linux (fp32, qnn_16a16w, qnn) / linux-job (gh) (trunk failure)
• pull / unittest / macos / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• pull / unittest-editable / macos / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

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