Graph: Added tf.peak_default_graph which just returns null if no default exists.

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

@@ -29,7 +29,19 @@ namespace Tensorflow
             return ops.get_default_graph();
         }
 
-        public Graph Graph() 
+        /// <summary>
+        ///     Equivalent to <see cref="get_default_graph"/> but does not create a new graph if it there is none.
+        /// </summary>
+        public Graph peak_default_graph()
+        {
+            return ops.default_graph_stack.peak_controller();
+        }
+
+        /// <summary>
+        ///     Creates a new graph.
+        /// </summary>
+        ///<remarks>Has no interaction with graph defaulting. Equivalent to new Graph();</remarks>
+        public Graph Graph()
             => new Graph();
     }
-}
+}

src/TensorFlowNET.Core/Graphs/DefaultGraphStack.cs

@@ -21,11 +21,10 @@ using static Tensorflow.Binding;
 
 namespace Tensorflow
 {
-
     /// <summary>
     ///     Serves as a stack for determining current default graph.
     /// </summary>
-    public class DefaultGraphStack 
+    public class DefaultGraphStack
     {
         private readonly List<StackModel> _stack = new List<StackModel>();
 
@@ -52,6 +51,20 @@ namespace Tensorflow
             throw new TensorflowException("Unable to find a default graph");
         }
 
+        public Graph peak_controller()
+        {
+            if (_stack.Count == 0 || _stack.Count(x => x.IsDefault) == 0)
+                return null;
+            for (var i = _stack.Count - 1; i >= 0; i--)
+            {
+                var x = _stack[i];
+                if (x.IsDefault)
+                    return x.Graph;
+            }
+
+            return null;
+        }
+
         public bool remove(Graph g)
         {
             if (_stack.Count == 0)