paddlepaddle平台上使用backward

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源代码：
d_on_cover = Critic(cover)
d_loss_cov = AdversaryLoss(d_on_cover, d_target_label_cover)
d_loss_cov.backward()

出现了报错：
framework.dygraph_tracer())
RuntimeError: (NotFound) Cannot find gradient of variable dygraph_tmp_397@GRAD
[Hint: Expected iter != accumulators.end() == true, but received iter != accumulators_.end():0 != true:1.] (at /paddle/paddle/fluid/imperative/basic_engine.cc:366)