fix: wire MoE topk via ixformer C++ bridge + disable broken flash_qla GDN

Two call chain breaks fixed:

1. MoE routing (2304 calls/token):
   BEFORE: torch.softmax + torch.topk (3 Python GPU ops, no ixformer)
   AFTER:  ix_bridge.py → ix_moe_bridge.cpp → ixformer::infer::topk_softmax()
   Source: upstream_ref/xllm/xllm/core/kernels/ilu/fused_moe.cpp line 46
   The C++ API exists in base image SDK (ixformer.h declares it),
   only the Python binding (ixformer.functions) was missing.

2. GDN prefill (4 layers, 99.98% NaN):
   BEFORE: flash_qla SM70 kernel → abs mean=inf → nan_to_num → zeros
   AFTER:  skip flash_qla, use _pytorch_forward directly
   Source: upstream_ref/xllm qwen3_gated_delta_net_base.cpp uses
   identical PyTorch chunked logic (no flash_qla).
   Sub168 (working build) never deployed flash_qla either.

Files:
- ex_engine/csrc/ix_moe_bridge.cpp: torch C++ extension calling ixformer C++ API
- ex_engine/python/ix_bridge.py: JIT-compile loader with PyTorch fallback
- qwen3_5.py: import ix_bridge for MoE, disable flash_qla for GDN
- patch_ops.sh: deploy ix_bridge .cpp + .py into vllm model dir

ex_engine/python/ix_bridge.py

@@ -0,0 +1,77 @@
+"""
+ix_bridge.py — Load ix_moe_bridge C++ extension at runtime.
+
+Calls ixformer::infer::topk_softmax() via C++ torch extension,
+bypassing the missing Python binding in ixformer.functions.
+
+Build: JIT-compiled on first import via torch.utils.cpp_extension.load()
+       (same mechanism as flash_qla_sm70 GDN kernel — proven to work on BI-V100)
+"""
+
+import os
+import logging
+import torch
+
+logger = logging.getLogger("ex_engine.ix_bridge")
+
+_ix_bridge = None
+_ix_bridge_available = False
+
+def _load_bridge():
+    """JIT-compile and load ix_moe_bridge.so"""
+    global _ix_bridge, _ix_bridge_available
+    if _ix_bridge is not None:
+        return _ix_bridge_available
+    
+    csrc_dir = os.path.join(os.path.dirname(os.path.dirname(os.path.abspath(__file__))), "csrc")
+    cpp_file = os.path.join(csrc_dir, "ix_moe_bridge.cpp")
+    
+    if not os.path.exists(cpp_file):
+        # Try deployed path (inside vllm model dir)
+        alt_dir = os.path.dirname(os.path.abspath(__file__))
+        cpp_file = os.path.join(alt_dir, "ix_moe_bridge.cpp")
+    
+    if not os.path.exists(cpp_file):
+        logger.warning("ix_moe_bridge.cpp not found at %s", cpp_file)
+        _ix_bridge_available = False
+        return False
+    
+    try:
+        from torch.utils.cpp_extension import load
+        logger.info("JIT-compiling ix_moe_bridge.cpp ...")
+        _ix_bridge = load(
+            name="ix_moe_bridge",
+            sources=[cpp_file],
+            extra_cflags=["-O2"],
+            verbose=False,
+        )
+        _ix_bridge_available = True
+        logger.info("ix_moe_bridge loaded successfully: %s", dir(_ix_bridge))
+        return True
+    except Exception as e:
+        logger.warning("ix_moe_bridge JIT compile failed: %s", e)
+        _ix_bridge_available = False
+        return False
+
+def topk_softmax(gating_output: torch.Tensor, topk: int, renormalize: bool = True):
+    """
+    Fused topk+softmax via ixformer C++ API.
+    
+    Args:
+        gating_output: (num_tokens, num_experts) router logits
+        topk: number of experts to select
+        renormalize: whether to renormalize weights
+    
+    Returns:
+        (topk_weights, topk_indices) — both (num_tokens, topk)
+    """
+    if not _ix_bridge_available:
+        if not _load_bridge():
+            # Fallback to pure PyTorch
+            probs = torch.softmax(gating_output.float(), dim=-1)
+            topk_w, topk_ids = torch.topk(probs, topk, dim=-1)
+            if renormalize:
+                topk_w = topk_w / topk_w.sum(dim=-1, keepdim=True)
+            return topk_w, topk_ids.to(torch.int32)
+    
+    return _ix_bridge.topk_softmax(gating_output, topk, renormalize)