distribute training crashed on develop branch

[Yancey1989]

Run the demo code test_word2vec.py with 2pservers + 2trainers, other demos can also reproduce this bug:

I0514 07:31:17.266929 11136 operator.cc:546] expected_kernel_key:data_type[float32]:data_layout[ANY_LAYOUT]:place[CPUPlace]:library_type[PLAIN]
PC: @                0x0 (unknown)
*** SIGSEGV (@0x0) received by PID 10770 (TID 0x7f8132309700) from PID 0; stack trace: ***
    @     0x7f83817d6390 (unknown)
    @     0x7f8334340dc7 std::_Hashtable<>::find()
    @     0x7f833433eb69 paddle::framework::Scope::FindVarLocally()
    @     0x7f833433eba4 paddle::framework::Scope::FindVar()
    @     0x7f833432aa1f paddle::framework::ExecutionContext::Input<>()
    @     0x7f8334230240 paddle::operators::SGDOpKernel<>::Compute()
    @     0x7f833432dac6 paddle::framework::OperatorWithKernel::RunImpl()
    @     0x7f833432a078 paddle::framework::OperatorBase::Run()
    @     0x7f8333b83ace paddle::framework::Executor::RunPreparedContext()

[seiriosPlus]

Run with 1pservers + 1trainers, pserver report this bug:

*** Error in `python': double free or corruption (out): 0x00007f771c000930 ***
======= Backtrace: =========
/lib/x86_64-linux-gnu/libc.so.6(+0x777e5)[0x7f778c9987e5]
/lib/x86_64-linux-gnu/libc.so.6(+0x8037a)[0x7f778c9a137a]
/lib/x86_64-linux-gnu/libc.so.6(cfree+0x4c)[0x7f778c9a553c]
/usr/local/lib/python2.7/dist-packages/paddle/fluid/core.so(_ZN6paddle9framework5Scope8DropKidsEv+0x39)[0x7f776ecc8c39]
/usr/local/lib/python2.7/dist-packages/paddle/fluid/core.so(_ZN6paddle9framework8Executor18RunPreparedContextEPNS0_22ExecutorPrepareContextEPNS0_5ScopeEbb+0x98a)[0x7f776e50baea]
/usr/local/lib/python2.7/dist-packages/paddle/fluid/core.so(+0xcaddf9)[0x7f776ebf6df9]
/usr/local/lib/python2.7/dist-packages/paddle/fluid/core.so(_ZNSt13__future_base13_State_baseV29_M_do_setEPSt8functionIFSt10unique_ptrINS_12_Result_baseENS3_8_DeleterEEvEEPb+0x2e)[0x7f776eb1204e]
/lib/x86_64-linux-gnu/libpthread.so.0(+0xea99)[0x7f778ccf9a99]
/usr/local/lib/python2.7/dist-packages/paddle/fluid/core.so(+0xcacf37)[0x7f776ebf5f37]
/usr/local/lib/python2.7/dist-packages/paddle/fluid/core.so(_ZN6paddle9framework10ThreadPool8TaskLoopEv+0x3f4)[0x7f776eccdf84]
/usr/lib/x86_64-linux-gnu/libstdc++.so.6(+0xb8c80)[0x7f778278cc80]
/lib/x86_64-linux-gnu/libpthread.so.0(+0x76ba)[0x7f778ccf26ba]
/lib/x86_64-linux-gnu/libc.so.6(clone+0x6d)[0x7f778ca2841d]

[Yancey1989]

Already fixed by #10663 , close this issue.