Exposing softmax activation

src/TensorFlowNET.Core/APIs/tf.nn.cs

@@ -116,6 +116,8 @@ namespace Tensorflow
             public IActivation relu() => new relu();
             public IActivation swish() => new swish();
             public IActivation tanh() => new tanh();
+
+            public IActivation softmax() => new softmax();
             public Tensor tanh(Tensor x, string name = null)
                 => gen_nn_ops.tanh(x, name);