use unique_ptr to hold m_allocator in the cpu backend

src/ngraph/runtime/cpu/cpu_backend.cpp

@@ -161,7 +161,7 @@ runtime::Allocator* runtime::cpu::CPU_Backend::get_host_memory_allocator()
 {
     if (m_allocator)
     {
-        return m_allocator;
+        return m_allocator.get();
     }
     else
     {
@@ -172,7 +172,7 @@ runtime::Allocator* runtime::cpu::CPU_Backend::get_host_memory_allocator()
 void runtime::cpu::CPU_Backend::set_host_memory_allocator(
     std::unique_ptr<runtime::Allocator> allocator)
 {
-    m_allocator = allocator.get();
+    m_allocator = std::move(allocator);
 }
 
 vector<runtime::PerformanceCounter> runtime::cpu::CPU_Executable::get_performance_data() const

src/ngraph/runtime/cpu/cpu_backend.hpp

@@ -70,7 +70,7 @@ namespace ngraph
             private:
                 std::unordered_map<std::shared_ptr<Function>, std::shared_ptr<Executable>>
                     m_exec_map;
-                Allocator* m_allocator;
+                std::unique_ptr<Allocator> m_allocator;
             };
 
             class CPU_BACKEND_API CPU_Executable : public runtime::Executable