Add W4A8 INT8 activation kernels for batched MoE prefill

[pytorchbot]

This PR was created by the merge bot to help merge the original PR into the main branch.
ghstack PR number: #19187 by @digantdesai
^ Please use this as the source of truth for the PR details, comments, and reviews
ghstack PR base: https://github.com/pytorch/executorch/tree/gh/digantdesai/50/base
ghstack PR head: https://github.com/pytorch/executorch/tree/gh/digantdesai/50/head
Merge bot PR base: https://github.com/pytorch/executorch/tree/main
Merge bot PR head: https://github.com/pytorch/executorch/tree/gh/digantdesai/50/orig

@diff-train-skip-merge

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/19226

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❌ 13 New Failures, 5 Pending, 4 Unrelated Failures

As of commit cfd120d with merge base cb4e5ae ():

NEW FAILURES - The following jobs have failed:

• MLX / test-mlx-parakeet / test-mlx-parakeet (gh)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 1
• pull / test-qnn-delegate-linux / linux-job (gh)
  backends/qualcomm/tests/test_qnn_delegate.py::TestQNNQuantizedOperator::test_qnn_conv1d_batch_norm
• pull / test-qnn-python-imports-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 5afb9c5e189a7594b753833cc1fb866c3c710d0df90dd2601e471a4cd2d00210 /exec failed with exit code 2
• Test CUDA Builds / export-model-cuda-artifact (nvidia, diar_streaming_sortformer_4spk-v2, non-quantized) / linux-job (gh)
  RuntimeError: Command docker exec -t 77911355dc61d607e2d715efd3f84fb1c7a960e6b7c822de21b064a40429018a /exec failed with exit code 1
• Test CUDA Builds / export-model-cuda-artifact (nvidia, parakeet-tdt, non-quantized) / linux-job (gh)
  RuntimeError: Command docker exec -t d5c216e1657b706bf7a673945cb7b816ee1665971b6587a1116c2e3f5cd93cb1 /exec failed with exit code 1
• Test CUDA Builds / export-model-cuda-artifact (nvidia, parakeet-tdt, quantized-int4-tile-packed) / linux-job (gh)
  RuntimeError: Command docker exec -t fa744b9b1c8963f0d25c8478c447c8122cfc4dadec5a9561747e68cf17dba041 /exec failed with exit code 1
• Test CUDA Windows Export and E2E / export-model-cuda-windows-artifact (facebook, dinov2-small-imagenet1k-1-layer, non-quantized) / linux-job (gh)
  RuntimeError: Command docker exec -t 7d84d7793fc43ebd9e04e0526b5bc1f6a484123102088b8e0523ab84f48b4b9f /exec failed with exit code 1
• Test CUDA Windows Export and E2E / export-model-cuda-windows-artifact (nvidia, parakeet-tdt, non-quantized) / linux-job (gh)
  RuntimeError: Command docker exec -t 262bd22cacf288947777ce0f6232c27ddb66267b752a6adae7465fd1b7d81c42 /exec failed with exit code 1
• Test CUDA Windows Export and E2E / export-model-cuda-windows-artifact (nvidia, parakeet-tdt, quantized-int4-weight-only) / linux-job (gh)
  RuntimeError: Command docker exec -t 3a13f6d5527f7cac9ecdb0c73b39cf12bb0b2aaf06cccf0f47ff1dc507691a0c /exec failed with exit code 1
• Test Metal Backend / export-model-metal-artifact (nvidia, parakeet-tdt, non-quantized) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• Test Metal Backend / export-model-metal-artifact (nvidia, parakeet-tdt, quantized-int4-metal) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• trunk / test-arm-backend-zephyr (ethos-u55) / linux-job (gh)
  RuntimeError: Command docker exec -t 7f92cce4b191b22f281aa868a186a38975acda7c32255b481f0add3fd4bc01f2 /exec failed with exit code 1
• trunk / test-arm-backend-zephyr (ethos-u85) / linux-job (gh)
  RuntimeError: Command docker exec -t 4f29a56e907bc62e260404d3661f31f1b0d1c5ec1fb5acd659c98e4f58e78781 /exec failed with exit code 1

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / test-qnn-testsuite-linux / test-backend-linux (qnn, models) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)

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

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

• MLX / test-mlx-qwen35-moe / test-mlx-qwen35-moe (gh) (trunk failure)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 1
• Test Metal Backend / test-metal-qwen35-moe-tiny / macos-job (gh) (trunk failure)
  /Users/ec2-user/runner/_work/executorch/executorch/pytorch/executorch/examples/models/qwen3_5_moe/main.cpp:65:16: error: use of undeclared identifier 'cudaSuccess'
• trunk / unittest-release / macos / macos-job (gh) (trunk failure)
  export/tests/test_target_recipes.py::TestTargetRecipes::test_mv2_model

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