Restore device-aware num_aie_columns in SwiGLU operators

[albiol2004]

Restores device-aware column selection in SwiGLUDecode and SwiGLUPrefill so both composite operators run on Phoenix (NPU1, 4 columns) as well as Strix (NPU2, 8 columns). The branching was originally introduced in #89 and inadvertently dropped during the simplifying refactor in #88, which hardcoded num_aie_columns=8 across both SwiGLU variants. This PR replaces the literal 8 with aie_utils.get_current_device().cols, matching the pattern already used by rms_norm, gemm, gemv, mem_copy, etc.

While re-threading the column count, num_aie_columns=n_cols is also now passed to the two GEMM calls in SwiGLUPrefill. Previously those defaulted to GEMM's fallback, which meant SwiGLU prefill was implicitly under-parallelized on NPU2 and misaligned with the column count used by the surrounding SiLU and ElementwiseMul sub-ops.

A new rectangular FFN shape (seq_len=256, embedding_dim=1024, hidden_dim=3584) is added to swiglu_prefill/test.py so real decoder-model FFN dimensions (e.g. Qwen3.5-0.8B) are exercised in CI alongside the existing square 2048² smoke test.

Added

• (256, 1024, 3584, False) rectangular FFN shape in iron/operators/swiglu_prefill/test.py, reflecting real decoder-model dims so non-square paths are covered.

Changed

• iron/operators/swiglu_decode/op.py: derive n_cols = aie_utils.get_current_device().cols and pass it to the gemv_1 / silu / eltwise_mul / gemv_2 sub-ops in place of the hardcoded 8.
• iron/operators/swiglu_prefill/op.py: same device-aware derivation, applied to silu / eltwise_mul and (newly) threaded through the gemm_1 and gemm_2 calls, which previously omitted num_aie_columns entirely.

Removed

• Hardcoded num_aie_columns=8 and associated // 8, // 16 literals in both SwiGLU op files.

Testing

Verified on NPU2 (Strix, aie2p) with ironenv + XRT sourced:

• pytest iron/operators/ -m "not extensive" --iterations 1 : all previously-passing tests still pass (pre-existing LeakyReLU skips unchanged).
• pytest iron/operators/swiglu_decode/test.py -v --iterations 1 : square 2048² passes.
• pytest iron/operators/swiglu_prefill/test.py -v --iterations 1 : both square 2048² and the new rectangular 256 × 1024 × 3584 pass.

NPU1 (Phoenix, aie2) hardware was not available for local validation; the column-selection logic is structurally identical to the previously-shipped #89 code path, so Phoenix behavior is expected to match that baseline and should be re-confirmed by a reviewer with Phoenix access.

During this work a separate, pre-existing numerical issue was observed in swiglu_decode at rectangular decode shapes (e.g. 1024 × 3584) that is unrelated to this change, the decode failure reproduces with num_aie_columns=8 either before or after this PR, so the rectangular case was not added to swiglu_decode/test.py in this PR. That issue is being investigated separately.

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