[executorch][gemma4] fuse MLP gate/up at GGUF load

[Gasoonjia]

Summary:
Move the gemma4 MLP gate_proj|up_proj fusion to a single backend-agnostic point in the GGUF loader, and make the model forward consume it. Supersedes the earlier CUDA-only export-time fusion (reverted here).

• gguf_loader.py: before any backend conversion (_convert_weight), buffer each layer's raw gate/up ExportableGGUFTensor and, once both arrive, row-concat their raw GGUF blocks along the output dim into one fused gate_up ExportableGGUFTensor (gate rows then up rows). Both backends then pack the
  already-fused weight with NO per-type concat: CUDA (Q4_K ->
  CudaCoalescedInt4Tensor, Q6_K -> CudaDp4aPlanarInt6Tensor) and MLX (ExportableGGUFTensor). Guards: same ggml_type + K; non-fuseable pairs and unpaired leftovers fall through unfused.
• Gemma4MLP: when a fused gate_up_proj is present, run one matmul and split the [.., 2*intermediate_size] output back into gate/up; otherwise use the separate projections. The shared MLP stays safe for unfused checkpoints and the prequant/HF load paths (no gate_up_proj -> original path, no crash).
• Revert the previous CUDA-localized fusion (cuda_source_transformations.py and export.py back to their original form). The kv_len-bounded tq4_sdpa kernel + call-site (already on main) are unchanged.

Single fusion point widens applicability (CUDA + MLX, incl. Q6_K) and keeps the model def backend-agnostic. Decode win is unchanged (same fused matmul, produced at load instead of at export).

Test Plan:

• Raw concat (real GGUF blk.0 ffn, q4_k): fused.dequantize() == [gate; up] stacked, bit-exact; fused CudaCoalescedInt4Tensor rows [:N]/[N:] qdata+scale+zero bit-identical to gate/up.
• Model-def fused vs unfused forward through real W4A8 int4_plain_mm: decode (T=1) bit-exact (cos 1.000000); prefill (T=4) cos 0.999988 -- the only delta is cuBLAS GEMM shape-dependent fp ordering (N=43008 vs 21504, identical weights), benign and inherent to any gate/up fusion.
• Full CUDA GGUF export (gemma4_31b, --turboquant, max-seq-len 131072): loader logs "Fused gate+up on 60 MLP layers", TurboQuant swaps 10 layers, AOTI build clean (model.pte + 26.18GB aoti_cuda_blob.ptd, "Done.").
• Decode via gemma4_31b_runner on the new build: coherent output, no NaN; prefill 1375 tok/s, decode 38.3 tok/s (no cuda_graph sanity).

[pytorch-bot]

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❌ 3 New Failures, 3 Unrelated Failures, 2 Unclassified Failures

As of commit 638f07a with merge base 65bc0ca ():

NEW FAILURES - The following jobs have failed:

• pull / test-arm-backend-no-driver (test_pytest_ops_tosa) / linux-job (gh)
  RuntimeError: Command docker exec -t 72cdd67f42e06d2ede8f80d8eb0aeee10db220d237655ea0413685dc83966c97 /exec failed with exit code 1
• pull / unittest / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 7583cc193407bd2707332fdbae7c4310bb34d72bc16464df5dfe1c96a8bd7554 /exec failed with exit code 1
• pull / unittest-editable / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 7fc4b5d7bf0ed1067764b993aa749177e7c7ff3f0b2f0f4225189d44ba1be7ee /exec failed with exit code 1

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
• Build Aarch64 Linux Wheels / pytorch/executorch / upload / upload-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)
  Unable to download artifact(s): Artifact not found for name: pytorch_executorch__3.10_cpu_aarch64

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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