Add CUDA sync + NaN/Inf check after each expert GEMM in grouped kernel

src/nvfp4_megamoe_kernel/cutlass_nvfp4_gemm/kernel.py

@@ -79,6 +79,15 @@ def cutlass_grouped_nvfp4_gemm(
             M_expert, N, K,
         )  # (M_expert, N) bfloat16
         
+        # Check for CUDA errors after each expert GEMM
+        err = torch.cuda.current_stream().synchronize()
+        
+        # Validate output
+        if torch.isnan(expert_out).any() or torch.isinf(expert_out).any():
+            if MEGA_MOE_DEBUG:
+                print(f"[cutlass_grouped_gemm] WARNING: expert {e} produced NaN/Inf, skipping")
+            continue
+        
         # Scatter back with routing weights
         for t_idx, token_idx in enumerate(token_indices):
             for k_idx in range(num_topk):