[ControlFlow] replace executor in run method of control flow ops with standalone_executor

[kangguangli]

PR types

Others

PR changes

OPs

Describe

The main content of this pr is to replace executor in run method of conditional block op with standalone_executor. The changes are mainly about two parts: ConditionalBlockOp and InterpreterCore.

1. Changes about ConditionalBlockOp
   1. replace Executor in RunImpl with InterpreterCore
   2. add class member std::shared_ptr<InterpreterCore> core to cache
   3. use flag FLAGS_control_flow_use_new_executor to decide whether use InterpreterCore
   4. no longer create new scope every times executing ConditionalBlockOp::RunImpl, only when the cached scope is invalid
   5. by default, InterpreterCore will rebuild and reset scope. The scope that was used to initialized it may be invalid, since it may be dropped by outer executor.
2. Changes about InterpreterCore
   1. add parameter used_for_control_flow_op for the initialization of InterpreterCore
   2. The executor in control flow does not need return value, so add a parameter need_fetch in InterpreterCore::Run
   3. interpretercore_utils.cc::BuildVariableScope should put persistent variables in global scope, which is the root of the scope tree.
   4. control flow block won't contains variables which occur in outer block, so these variables should be skipped in gc.
   5. for interpretercore_util.cc::BuildVariableMap, some inputs or outputs of op in the control flow block is recorded in outer block, so the corresponding variables should be queried recursively.
3. Other changes
   1. sometimes the layout of variable will be changed after first execution of executor, so there's no need to execute transfer_layout really. We simply call memcpy instead.

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[zhiqiu]

LGTM

[Aurelius84]

LGTM