Port ClGemmLowpOutputStage operator to new interface

Signed-off-by: Georgios Pinitas <georgios.pinitas@arm.com>
Change-Id: I96630b45887eaba16ef358b95f3d9ac0b9045157
Reviewed-on: https://review.mlplatform.org/c/ml/ComputeLibrary/+/5882
Tested-by: Arm Jenkins <bsgcomp@arm.com>
Reviewed-by: Michele Di Giorgio <michele.digiorgio@arm.com>
Comments-Addressed: Arm Jenkins <bsgcomp@arm.com>
diff --git a/arm_compute/runtime/CL/functions/CLGEMMLowpOutputStage.h b/arm_compute/runtime/CL/functions/CLGEMMLowpOutputStage.h
index e85f2db..6ec7b71 100644
--- a/arm_compute/runtime/CL/functions/CLGEMMLowpOutputStage.h
+++ b/arm_compute/runtime/CL/functions/CLGEMMLowpOutputStage.h
@@ -112,10 +112,8 @@
     void run() override;
 
 private:
-    std::unique_ptr<ICLKernel> _kernel;
-    const ICLTensor           *_input;
-    const ICLTensor           *_bias;
-    ICLTensor                 *_output;
+    struct Impl;
+    std::unique_ptr<Impl> _impl;
 };
 } // namespace arm_compute
 #endif /*ARM_COMPUTE_CLGEMMLOWPOUTPUTSTAGE_H */