IVGCVSW-1964 : replace optional biases with home-grown Optional

!android-nn-driver:151788

Change-Id: Ibdc41d09b8df05e7a0360dcb8a060860dfb1bd99
diff --git a/src/backends/cl/workloads/ClDepthwiseConvolutionBaseWorkload.hpp b/src/backends/cl/workloads/ClDepthwiseConvolutionBaseWorkload.hpp
index 9d5cde3..27aec8e 100644
--- a/src/backends/cl/workloads/ClDepthwiseConvolutionBaseWorkload.hpp
+++ b/src/backends/cl/workloads/ClDepthwiseConvolutionBaseWorkload.hpp
@@ -6,7 +6,6 @@
 #pragma once
 
 #include <backends/Workload.hpp>
-#include <boost/optional.hpp>
 
 #include <arm_compute/runtime/CL/CLFunctions.h>
 
@@ -17,7 +16,7 @@
                                                            const TensorInfo& output,
                                                            const DepthwiseConvolution2dDescriptor& descriptor,
                                                            const TensorInfo& weights,
-                                                           const boost::optional<TensorInfo>& biases);
+                                                           const Optional<TensorInfo>& biases);
 
 template<armnn::DataType... dataTypes>
 class ClDepthwiseConvolutionBaseWorkload : public TypedWorkload<DepthwiseConvolution2dQueueDescriptor, dataTypes...>