Implement FuncGraph.capture_eager_tensor.

Author: Oceania2018

src/TensorFlowNET.Core/Graphs/FuncGraph.cs

@@ -140,7 +140,32 @@ public Tensor capture(Tensor tensor, string name = null, TensorShape shape = nul
         }
 
         Tensor capture_eager_tensor(Tensor tensor, string name)
-            => throw new NotImplementedException("");
+        {
+            Tensor graph_const = null;
+            if (!_captures.ContainsKey(tensor.Id))
+            {
+                graph_const = tf_with(ops.control_dependencies(null), ctl
+                    => constant_op.constant(tensor.numpy(), dtype: tensor.dtype, shape: tensor.shape, name: name));
+                add_capture(tensor, graph_const);
+            }
+            else
+            {
+                graph_const = _captures[tensor.Id].Item2;
+            }
+
+            BackwardFunction _backward_function_wrapper = (output_grads, unneeded_gradients) =>
+            {
+                return output_grads;
+            };
+
+            tf.Runner.RecordGradient("captured_value",
+                new[] { graph_const }, null,
+                new[] { tensor },
+                getBackwardFunction: () => _backward_function_wrapper
+                /*getForwardFunction: forward_function*/);
+
+            return graph_const;
+        }
 
         Tensor _capture_helper(Tensor tensor, string name, TensorShape shape = null)
         {