Created by: DannyIsFunny

【Issue】 Unknown segmentation error may occur when we use multi-thread to create many predictors at the same time. eg.

#include <iostream>
#include <vector>
#include "paddle_api.h"  // NOLINT
#include <unistd.h>
#include <pthread.h>

using namespace paddle::lite_api;  // NOLINT

#define NUM_THREADS     2

struct thread_data{
   int  thread_id;
   char *message;
};

int64_t ShapeProduction(const shape_t& shape) {
  int64_t res = 1;
  for (auto i : shape) res *= i;
  return res;
}
void* RunModel(void*arg) {
  for(int i =0; i < 10; i++) {
    // 1. Create CxxConfig
    std::string model_dir = "mobilenet_v1";
    CxxConfig config;
    config.set_model_dir(model_dir);
    config.set_valid_places({Place{TARGET(kX86), PRECISION(kFloat)},
                             Place{TARGET(kHost), PRECISION(kFloat)}});
    // 2. Create PaddlePredictor by CxxConfig
    std::shared_ptr<PaddlePredictor> predictor1 =
        CreatePaddlePredictor<CxxConfig>(config);
    std::cout << "Succeed!" << std::endl;
  }
    return 0;
}

int main(int argc, char** argv) {
  if (argc < 2) {
    std::cerr << "[ERROR] usage: ./" << argv[0] << " naive_buffer_model_dir\n";
    exit(1);
  }
  std::string model_dir = argv[1];

   pthread_t threads[NUM_THREADS];
   struct thread_data td[NUM_THREADS];
   int rc;
   int i;

   for( i=0; i < NUM_THREADS; i++ ){
      std::cout <<"main() : creating thread, " << i << std::endl;
      td[i].thread_id = i;
      td[i].message = (char*)"This is message";
      rc = pthread_create(&threads[i], NULL,
                          RunModel, NULL);
      if (rc){
         std::cout << "Error:unable to create thread," << rc << std::endl;
         exit(-1);
      }
   }
   pthread_exit(NULL);

  return 0;
}

【Reason】CreatePaddlePredictor method is not safe for multi-thread condition. Further more, we have located that inside optimizer->GenRuntimeProgram() has caused this problem. 【Effect of Current PR】 Add thread lock to CreatePaddlePredictor, so that CreatePaddlePredictor method will be operated sequently in multi-thread program to avoid unknown error.