enable unit test for test_nce

test=develop

python/paddle/fluid/layers/tensor.py

@@ -314,6 +314,7 @@ def assign(input, output=None, init_once=False):
                 shape=input.shape,
                 dtype=input.dtype,
                 default_initializer=Constant(0.0))
+            output.stop_gradient = True
         else:
             output = helper.create_variable_for_type_inference(
                 dtype=input.dtype)

python/paddle/fluid/tests/unittests/CMakeLists.txt

@@ -32,7 +32,6 @@ endif()
 list(REMOVE_ITEM TEST_OPS test_seq_concat_op) # FIXME(helin): https://github.com/PaddlePaddle/Paddle/issues/8290
 list(REMOVE_ITEM TEST_OPS test_modified_huber_loss_op) # FIXME(qijun) https://github.com/PaddlePaddle/Paddle/issues/5184
 list(REMOVE_ITEM TEST_OPS test_lstm_unit_op) # # FIXME(qijun) https://github.com/PaddlePaddle/Paddle/issues/5185
-list(REMOVE_ITEM TEST_OPS test_nce) # FIXME(qijun) https://github.com/PaddlePaddle/Paddle/issues/7778
 list(REMOVE_ITEM TEST_OPS test_recurrent_op) # FIXME(qijun) https://github.com/PaddlePaddle/Paddle/issues/6152
 list(REMOVE_ITEM TEST_OPS test_cond_op) # FIXME(qijun): https://github.com/PaddlePaddle/Paddle/issues/5101#issuecomment-339814957