add _pop_tape.

src/TensorFlowNET.Core/Gradients/GradientActor.cs

@@ -48,6 +48,14 @@ namespace Tensorflow.Gradients
             _recording = true;
         }
 
+        private void _pop_tape()
+        {
+            if (!_recording)
+                throw new ValueError("Tape is not recording.");
+            _tape.pop_tape(_tape);
+            _recording = false;
+        }
+
         /// <summary>
         /// Marks this tensor to be watched by the given tape.
         /// </summary>
@@ -59,12 +67,19 @@ namespace Tensorflow.Gradients
 
         public Tensor gradient(Tensor target, Tensor sources)
         {
-            using (var status = new Status())
+            if(_recording)
             {
-                c_api.TFE_TapeGradient(_tape, new IntPtr[] { target }, IntPtr.Zero, status);
+                if (!_persistent)
+                    _pop_tape();
             }
-                
-            return null;
+
+            using var status = new Status();
+            var et = c_api.TFE_TapeGradient(_tape, 
+                new IntPtr[] { (target as EagerTensor).EagerTensorHandle }, 1,
+                new IntPtr[] { (sources as EagerTensor).EagerTensorHandle }, 1,
+                status);
+            status.Check(true);
+            return et;
         }
 
         public void Dispose()

src/TensorFlowNET.Core/Gradients/Tape.cs

@@ -20,6 +20,11 @@ namespace Tensorflow.Gradients
             c_api.TFE_TapeWatch(_handle, x.EagerTensorHandle);
         }
 
+        public void pop_tape(Tape tape)
+        {
+            c_api.TFE_TapeSetRemove(tape);
+        }
+
         public static bool IsDtypeTrainable(DataType dtype)
         {
             switch (dtype)

src/TensorFlowNET.Core/Operations/gen_math_ops.cs

@@ -715,17 +715,15 @@ namespace Tensorflow
         {
             if (tf.context.executing_eagerly())
             {
-                using (var status = new Status())
+                using var status = new Status();
+                var _result = c_api.TFE_FastPathExecute(tf.context, tf.context.device_name,
+                "Mul", name, new IntPtr[]
                 {
-                    var _result = c_api.TFE_FastPathExecute(tf.context, tf.context.device_name,
-                    "Mul", name, new IntPtr[]
-                    {
-                        (x as EagerTensor).EagerTensorHandle,
-                        (y as EagerTensor).EagerTensorHandle
-                    }, 2, status);
-                    status.Check(true);
-                    return new EagerTensor(_result);
-                }
+                    (x as EagerTensor).EagerTensorHandle,
+                    (y as EagerTensor).EagerTensorHandle
+                }, 2, status);
+                status.Check(true);
+                return new EagerTensor(_result);
             }
 
             var _op = _op_def_lib._apply_op_helper("Mul", name, args: new { x, y });