[CI] Fix conch kernel crash on 3D input by reshaping to 2D before GEMM (#38178)

Signed-off-by: Andreas Karatzas <akaratza@amd.com>

vllm/model_executor/kernels/linear/mixed_precision/conch.py

@@ -134,8 +134,11 @@ class ConchLinearKernel(MPLinearKernel):
         if group_size == -1:
             group_size = x.shape[-1]
 
+        x_2d = x.reshape(-1, x.shape[-1])
+        out_shape = x.shape[:-1] + (self.config.partition_weight_shape[1],)
+
         output = mixed_precision_gemm(
-            x=x,
+            x=x_2d,
             w_q_packed=w_q.data,
             w_s=w_s.data,
             w_zp=w_zp.data if w_zp is not None else None,
@@ -147,4 +150,4 @@ class ConchLinearKernel(MPLinearKernel):
         if bias is not None:
             output.add_(bias)  # In-place add
 
-        return output
+        return output.reshape(out_shape)