IVGCVSW-4439: Adding Elu support to Activation

 * Added CpuRef implementation
 * Added Unit Tests
 * Added Quantizer Test
 * Enabled Tests for Neon and CL backends on fp32 only
 * Added to Serializer

Signed-off-by: David Monahan <david.monahan@arm.com>
Change-Id: Ic23e1797dbc9352b40678c389d7fe2b836b582ea
diff --git a/src/backends/reference/workloads/Activation.cpp b/src/backends/reference/workloads/Activation.cpp
index 814a0dd..82dd919 100644
--- a/src/backends/reference/workloads/Activation.cpp
+++ b/src/backends/reference/workloads/Activation.cpp
@@ -69,6 +69,11 @@
             output = a * tanhf(b * in);
             break;
         }
+        case ActivationFunction::Elu:
+        {
+            output = (in >= 0) ? in : a * (expf(in) - 1);
+            break;
+        }
         default:
         {
             throw InvalidArgumentException("Unsupported activation function");