Handle case when delement is None in PopBackGrad.

Fixes https://github.com/tensorflow/tensorflow/issues/37230

PiperOrigin-RevId: 303845628
Change-Id: Ia0159cb2dfbc70112f822f17e88182e414a83494

tensorflow/python/eager/pywrap_gradient_exclusions.cc

@@ -410,7 +410,7 @@ absl::optional<tensorflow::gtl::FlatSet<int>> OpGradientUnusedInputIndices(
 
 absl::optional<tensorflow::gtl::FlatSet<int>> OpGradientUnusedOutputIndices(
     const tensorflow::string &op_name) {
-  static std::array<OpIndexInfo, 460> a = {{
+  static std::array<OpIndexInfo, 459> a = {{
       {"Abs"},
       {"AccumulateNV2"},
       {"Acos"},
@@ -833,7 +833,6 @@ absl::optional<tensorflow::gtl::FlatSet<int>> OpGradientUnusedOutputIndices(
       {"TensorListGather"},
       {"TensorListGetItem"},
       {"TensorListLength"},
-      {"TensorListPopBack", 1, {1}},
       {"TensorListPushBack"},
       {"TensorListPushBackBatch"},
       {"TensorListResize"},

tensorflow/python/kernel_tests/list_ops_test.py

@@ -1665,6 +1665,26 @@ class ListOpsTest(test_util.TensorFlowTestCase, parameterized.TestCase):
 
     self.assertAllEqual(f(), [b"A", b"B", b"C"])
 
+  def testPopBackGrad(self):
+    # https://github.com/tensorflow/tensorflow/issues/37230
+
+    @def_function.function
+    def g(x):
+      x_prod = constant_op.constant([1.])
+      for unused_i in math_ops.range(3):
+        x_prod = x_prod * x
+      return x_prod
+
+    x = constant_op.constant(1.)
+    with backprop.GradientTape() as t:
+      t.watch(x)
+      with backprop.GradientTape() as tt:
+        tt.watch(x)
+        loss = g(x)
+      jac = tt.gradient(loss, x)
+    hess = t.gradient(jac, x)
+    self.assertAllEqual(hess, 6.)
+
 
 if __name__ == "__main__":
   test.main()

tensorflow/python/ops/list_ops.py

@@ -186,6 +186,8 @@ def _PopBackGrad(op, dlist, delement):
         element_dtype=delement.dtype,
         element_shape=gen_list_ops.tensor_list_element_shape(
             op.outputs[0], shape_type=dtypes.int32))
+  if delement is None:
+    delement = array_ops.zeros_like(op.outputs[1])
   return gen_list_ops.tensor_list_push_back(dlist, delement), None