GC.Collect in ComputeGradient

src/TensorFlowNET.Core/Gradients/Tape.ComputeGradient.cs

@@ -26,6 +26,7 @@ namespace Tensorflow.Gradients
                 tensor_tape_,
                 state.op_tape);
 
+            int gcCollectFlag = 0;
             while (!op_stack.empty())
             {
                 var op = op_stack.Dequeue();
@@ -154,6 +155,9 @@ namespace Tensorflow.Gradients
                             op_stack.Enqueue(op_id);
                     }
                 }
+
+                if (gcCollectFlag++ % 10 == 0)
+                    GC.Collect();
             }
 
             if (state.op_tape.Count > 0)