利用train_from_dataset进行训练的时候，第一个epoch正常，到了第二个epoch就报错了，好像是跟thread有关。
系统信息：centos，fluid1.6 cpu版本。

输出如下：

==== epoch_id ==== 0
loss    reduce_mean_0.tmp_0             lod: {}
        dim: 1
        layout: NCHW
        dtype: float
        data: [1.70779]

==== finished ====
==== epoch_id ==== 1
W1106 10:26:39.796146  5593 init.cc:212] *** Aborted at 1573007199 (unix time) try "date -d @1573007199" if you are using GNU date ***
W1106 10:26:39.798044  5593 init.cc:212] PC: @                0x0 (unknown)
W1106 10:26:39.798264  5593 init.cc:212] *** SIGSEGV (@0x2) received by PID 5593 (TID 0x7ff678c76700) from PID 2; stack trace: ***
W1106 10:26:39.799917  5593 init.cc:212]     @     0x7ff679740160 (unknown)
W1106 10:26:39.801622  5593 init.cc:212]     @     0x7ff678cf47d4 __GI___libc_free
W1106 10:26:39.803226  5593 init.cc:212]     @     0x7ff67973873a __pthread_create_2_1
W1106 10:26:39.804966  5593 init.cc:212]     @     0x7ff679a61aed PyThread_start_new_thread
W1106 10:26:39.806640  5593 init.cc:212]     @     0x7ff679a65f87 thread_PyThread_start_new_thread
W1106 10:26:39.808364  5593 init.cc:212]     @     0x7ff679a34ea4 PyEval_EvalFrameEx
W1106 10:26:39.810075  5593 init.cc:212]     @     0x7ff679a3517e PyEval_EvalFrameEx
W1106 10:26:39.811789  5593 init.cc:212]     @     0x7ff679a3517e PyEval_EvalFrameEx
W1106 10:26:39.813552  5593 init.cc:212]     @     0x7ff679a366c9 PyEval_EvalCodeEx
W1106 10:26:39.815271  5593 init.cc:212]     @     0x7ff679a33b98 PyEval_EvalFrameEx
W1106 10:26:39.816990  5593 init.cc:212]     @     0x7ff679a366c9 PyEval_EvalCodeEx
W1106 10:26:39.818692  5593 init.cc:212]     @     0x7ff679a33b98 PyEval_EvalFrameEx
W1106 10:26:39.820422  5593 init.cc:212]     @     0x7ff679a3517e PyEval_EvalFrameEx
W1106 10:26:39.822183  5593 init.cc:212]     @     0x7ff679a366c9 PyEval_EvalCodeEx
W1106 10:26:39.823884  5593 init.cc:212]     @     0x7ff679a33b98 PyEval_EvalFrameEx
W1106 10:26:39.825582  5593 init.cc:212]     @     0x7ff679a366c9 PyEval_EvalCodeEx
W1106 10:26:39.827316  5593 init.cc:212]     @     0x7ff679a33b98 PyEval_EvalFrameEx
W1106 10:26:39.829082  5593 init.cc:212]     @     0x7ff679a366c9 PyEval_EvalCodeEx
W1106 10:26:39.830780  5593 init.cc:212]     @     0x7ff679a368ea PyEval_EvalCode
W1106 10:26:39.832540  5593 init.cc:212]     @     0x7ff679a4fbad run_mod
W1106 10:26:39.834237  5593 init.cc:212]     @     0x7ff679a50d28 PyRun_FileExFlags
W1106 10:26:39.835966  5593 init.cc:212]     @     0x7ff679a51f48 PyRun_SimpleFileExFlags
W1106 10:26:39.837720  5593 init.cc:212]     @     0x7ff679a6416c Py_Main
W1106 10:26:39.839462  5593 init.cc:212]     @     0x7ff678c99bd5 __libc_start_main
W1106 10:26:39.841110  5593 init.cc:212]     @     0x7ff679b3107f (unknown)