[qwen] Add Qwen3_5MoeForCausalLM (flat text-only MoE) support

[Copilot]

Propagates onnxruntime-genai#2146 (Qwen3.5‑MoE). The MoE *ForConditionalGeneration builder from #2146 was already present and faithful to upstream; the gap was the flat text-only Qwen3_5MoeForCausalLM variant, which mbext provides for the dense model (Qwen35CausalLMModel/Qwen3_5ForCausalLM) but not for MoE.

Changes

• builders/qwen.py
  • Generalized Qwen35MoeTextModel.__init__ so the MoE config mapping (num_experts → num_local_experts, moe_intermediate_size → intermediate_size) applies to both the nested (text_config) and the flat Qwen3_5MoeTextConfig:

    tc = config.text_config if hasattr(config, "text_config") else config

  • Added Qwen35MoeCausalLMModel(Qwen35MoeTextModel), mirroring the dense Qwen35CausalLMModel: defaults exclude_embeds=False (keeps the embedding layer so the graph takes input_ids) and loads weights via Qwen3_5MoeForCausalLM.
• builder.py — dispatch Qwen3_5MoeForCausalLM → Qwen35MoeCausalLMModel.
• tests/fast/test_random_qwen3_5_moe.py — flat-config helper + test_qwen3_5_moe_causallm_fp32_cpu_full_attention_build asserting input_ids input, an MoE/QMoE op, and model_type == "qwen3_5_moe_text".

Notes for reviewers

Qwen3_5MoeForCausalLM is an mbext-side extension (not in #2146 upstream), added for parity with the existing dense ForCausalLM path. The text-only model_type (qwen3_5_moe_text) aligns with the model_type.h entry from onnxruntime-genai#2186.

[github-actions]

📂 Impacted Files

3 file(s) changed in this pull request:

🔵 modelbuilder/builder.py (+4 / -0)
🔵 modelbuilder/builders/qwen.py (+42 / -8)
🔵 tests/fast/test_random_qwen3_5_moe.py (+116 / -0)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.91%. Comparing base (65b1a42) to head (20673a5).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #384      +/-   ##
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+ Coverage   78.88%   78.91%   +0.02%     
==========================================
  Files          27       27              
  Lines        8113     8123      +10     
==========================================
+ Hits         6400     6410      +10     
  Misses       1713     1713              

Flag	Coverage Δ
fast-tests	78.91% <100.00%> (+0.02%)	⬆️
fast-tests-ort-nightly	78.86% <100.00%> (+0.02%)	⬆️

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