COMPMID-556 Add macro for checking graph's tensor object

Change-Id: I1de1c2932c9906b23b49e18bd82c173846d98723
Reviewed-on: http://mpd-gerrit.cambridge.arm.com/93860
Tested-by: Kaizen <jeremy.johnson+kaizengerrit@arm.com>
Reviewed-by: Anthony Barbier <anthony.barbier@arm.com>
Reviewed-by: Pablo Tello <pablo.tello@arm.com>
diff --git a/arm_compute/graph/SubTensor.h b/arm_compute/graph/SubTensor.h
index 22a0a9e..72aa789 100644
--- a/arm_compute/graph/SubTensor.h
+++ b/arm_compute/graph/SubTensor.h
@@ -72,9 +72,10 @@
     bool                  call_accessor() override;
     bool                  has_accessor() const override;
     arm_compute::ITensor *set_target(TargetHint target) override;
-    arm_compute::ITensor *tensor() override;
-    TargetHint            target() const override;
-    void                  allocate() override;
+    arm_compute::ITensor       *tensor() override;
+    const arm_compute::ITensor *tensor() const override;
+    TargetHint                  target() const override;
+    void                        allocate() override;
 
 private:
     /** Instantiates a sub-tensor */