Fix the circular reference of GradientTape

Summary:
This commit removes the unused reference of GradientTape to
avoid the circular reference issue.

dragon/operators/array/identity_op.cc

+#include "dragon/core/workspace.h"
 #include "dragon/operators/array/reshape_ops.h"
 #include "dragon/utils/math_functions.h"
 
@@ -17,6 +18,11 @@ DEPLOY_CPU_OPERATOR(Identity);
 DEPLOY_CUDA_OPERATOR(Identity);
 #endif
 
+DEPLOY_CPU_OPERATOR(IdentityGradient);
+#ifdef USE_CUDA
+DEPLOY_CUDA_OPERATOR(IdentityGradient);
+#endif
+
 OPERATOR_SCHEMA(Identity)
     /* X */
     .NumInputs(1)

dragon/python/core/eager/backprop.py

@@ -30,9 +30,10 @@ from dragon.core.util import tls
 
 
 class Tape(object):
-    def __init__(self, parent):
+    """Tape instance."""
+
+    def __init__(self):
         self._defs = []
-        self._parent = parent
         self._watched = set()
         self._empty_grads = set()
         self._gc = workspace.get_workspace().collectors
@@ -137,6 +138,8 @@ class GradientTape(object):
             raise RuntimeError(
                 'GradientTape.gradient(...) can only be called '
                 'once on non-persistent tapes.')
+
+        # Stop recording if not persistent.
         if self._recording:
             if not self._persistent:
                 self._pop_tape()
@@ -212,7 +215,7 @@ class GradientTape(object):
         if self._recording:
             raise ValueError('Tape is already recording.')
         if self._tape is None:
-            self._tape = Tape(self)
+            self._tape = Tape()
         push_new_tape(self._tape)
         self._recording = True
 

test/dragon/test_ops.py

@@ -660,8 +660,11 @@ class TestArrayOps(OpTestCase):
             with execution_context().mode(execution):
                 data = arange((4,))
                 x = new_tensor(data)
-                y = dragon.identity(x)
-                self.assertEqual(y, data)
+                with dragon.GradientTape() as tape:
+                    tape.watch(x)
+                    y = dragon.identity(x)
+                dx = tape.gradient(y, [x], output_gradients=[x])[0]
+                self.assertEqual([y, dx], [data, data])
 
     @unittest.skipIf(not TEST_CUDA, 'CUDA unavailable')
     def test_identity_cuda(self):