fix is_build_function in Layer.

Author: Oceania2018

src/TensorFlowNET.Core/Contexts/Context.cs

@@ -42,7 +42,7 @@ public Context(ContextOptions opts, Status status)
         {
             Handle = c_api.TFE_NewContext(opts.Handle, status.Handle);
             status.Check(true);
-            context_switches = new ContextSwitchStack(defaultExecutionMode == EAGER_MODE);
+            context_switches = new ContextSwitchStack(defaultExecutionMode == EAGER_MODE, false);
             initialized = true;
         }
 
@@ -70,21 +70,19 @@ public void end_step()
         public bool executing_eagerly()
             => context_switches.Current().EagerMode;
 
+        public bool is_build_function()
+            => context_switches.Current().IsBuildingFunction;
+
         public string shared_name(string name = null)
             => !string.IsNullOrEmpty(name) || !executing_eagerly() ?
                 name :
                 "cd2c89b7-88b7-44c8-ad83-06c2a9158347";
 
-        public void graph_mode()
-            => mode(false);
-
-        public void eager_mode()
-            => mode(true);
+        public void graph_mode(bool isFunc = false)
+            => context_switches.Push(false, isFunc);
 
-        void mode(bool isEager)
-        {
-            context_switches.Push(isEager);
-        }
+        public void eager_mode(bool isFunc = false)
+            => context_switches.Push(true, isFunc);
 
         public void restore_mode()
         {

src/TensorFlowNET.Core/Contexts/ContextSwitchStack.cs

@@ -25,17 +25,18 @@ public class ContextSwitchStack
     {
         Stack<ContextSwitch> stack;
 
-        public ContextSwitchStack(bool isEager)
+        public ContextSwitchStack(bool isEager, bool isFunc)
         {
             stack = new Stack<ContextSwitch>();
-            Push(isEager);
+            Push(isEager, isFunc);
         }
 
-        public void Push(bool isEager)
+        public void Push(bool isEager, bool isFunc)
         {
             stack.Push(new ContextSwitch
             {
-                EagerMode = isEager
+                EagerMode = isEager,
+                IsBuildingFunction = isFunc
             });
         }
 

src/TensorFlowNET.Keras/Engine/Layer.FunctionalConstructionCall.cs

@@ -25,7 +25,7 @@ Tensors FunctionalConstructionCall(Tensors inputs)
             // using var graph = tf.keras.backend.get_graph().as_default();
 
             if (!inputs.IsEagerTensor)
-                tf.Context.graph_mode();
+                tf.Context.graph_mode(isFunc: true);
 
             tf_with(ops.name_scope(_name_scope()), scope =>
             {

src/TensorFlowNET.Keras/Engine/Layer.cs

@@ -176,9 +176,17 @@ protected void MaybeBuild(Tensors inputs)
 
             tf.init_scope();
 
-            tf.Context.eager_mode();
+            bool need_restore_mode = false;
+            if (inputs.IsEagerTensor || tf.Context.is_build_function())
+            {
+                need_restore_mode = true;
+                tf.Context.eager_mode();
+            }
+               
             build(inputs);
-            tf.Context.restore_mode();
+
+            if (need_restore_mode)
+                tf.Context.restore_mode();
 
             built = true;
         }