Need to flush the program desc to update the proto information

[Xreki]

In Python api, there are many interfaces to modify the program and block, such as append_op,
insert_op, remove_op, delete_ops, etc. Corresponding C++ functions will be called to update the framework::BlockDescs in framework::ProgramDesc. In these implementations, a member variable need_update_ is set to true, which means the proto::BlockDesc is not consistant with the framework::BlockDesc, which means the need to update before using.

Paddle/paddle/fluid/pybind/pybind.cc

Lines 295 to 301 in 879b7c5

	ProgramDesc prog_with_targets(origin);
	for (const auto &t : targets) {
	prog_with_targets.MutableBlock(t[0])->Op(t[1])->SetIsTarget(true);
	}
	proto::ProgramDesc pruned_desc;
	Prune(*prog_with_targets.Proto(), &pruned_desc);
	return new ProgramDesc(pruned_desc);

In above codes, ProgramDesc origin is used to define a new ProgramDesc. If the proto information of origin is not consistant with itself, following error may happen:

*** Aborted at 1524129285 (unix time) try "date -d @1524129285" if you are using GNU date ***
PC: @                0x0 (unknown)
*** SIGSEGV (@0x8) received by PID 28771 (TID 0x7f7dc7bae700) from PID 8; stack trace: ***
    @       0x318b20f500 (unknown)
    @     0x7f7d92eaf6c0 paddle::framework::proto::OpDesc::UnsafeMergeFrom()
    @     0x7f7d92eb081f paddle::framework::proto::BlockDesc::UnsafeMergeFrom()
    @     0x7f7d92eb0bdb paddle::framework::proto::ProgramDesc::UnsafeMergeFrom()
    @     0x7f7d90d83c86 paddle::framework::ProgramDesc::ProgramDesc()
    @     0x7f7d90cf94cb _ZZN8pybind1112cpp_function10initializeIZN6paddle6pybindL13pybind11_initEvEUlRKNS2_9framework11ProgramDescERKSt6vectorISt5arrayImLm2EESaISA_EEE37_PS5_IS7_SE_EINS_4nameENS_5scopeENS_7siblingEEEEvOT_PFT0_DpT1_EDpRKT2_ENUlRNS_6detail13function_callEE1_4_FUNESX_
    @     0x7f7d90d0e81a pybind11::cpp_function::dispatcher()
    @     0x7f7dc84ba649 PyEval_EvalFrameEx
    @     0x7f7dc84bc130 PyEval_EvalCodeEx
    @     0x7f7dc84ba4a1 PyEval_EvalFrameEx
    @     0x7f7dc84bc130 PyEval_EvalCodeEx
    @     0x7f7dc84ba4a1 PyEval_EvalFrameEx
    @     0x7f7dc84bac56 PyEval_EvalFrameEx
    @     0x7f7dc84bac56 PyEval_EvalFrameEx
    @     0x7f7dc84bc130 PyEval_EvalCodeEx
    @     0x7f7dc84bc242 PyEval_EvalCode
    @     0x7f7dc84d662c run_mod
    @     0x7f7dc84d6700 PyRun_FileExFlags
    @     0x7f7dc84d7c0c PyRun_SimpleFileExFlags
    @     0x7f7dc84e94cc Py_Main
    @     0x7f7dc7bd1185 __libc_start_main

Sometimes, after we change the program in Python codes, we need to call flush explicitly.

[kexinzhao]

I encountered this issue when I am debugging the float16 transpiler. Thanks for bringing this issue up!