use cross validation in homo_nn

[wangt97]

Describe the bug

                "cv_param": {
                    "n_splits": 4,
                    "shuffle": true,
                    "random_seed": 33,
                    "need_cv": true
                },

add above config in official examples of homo_nn, when running the training, a error occurred as below

[DEBUG] [2022-07-12 00:32:15,009] [202207120030337870380] [26410:139637714597696] - [model_base._run] [line:234]: running_funcs: [<bound method ModelBase.cross_validation of <federatedml.nn.homo_nn.enter_point.HomoNNDefaultClient object at 0x7eff71ea8fd0>>]
[DEBUG] [2022-07-12 00:32:15,010] [202207120030337870380] [26410:139637714597696] - [model_base._run] [line:257]: saved_result is : [None], data_output: None
[INFO] [2022-07-12 00:32:15,010] [202207120030337870380] [26410:139637714597696] - [tracker_client.log_component_summary] [line:288]: Request save job 202207120030337870380 task 202207120030337870380_homo_nn_0 0 on guest 9999 component summary
[ERROR] [2022-07-12 00:32:15,059] [202207120030337870380] [26410:139637714597696] - [task_executor._run_] [line:243]: 'NoneType' object has no attribute 'export_model'
Traceback (most recent call last):
  File "./fate/fateflow/python/fate_flow/worker/task_executor.py", line 195, in _run_
    cpn_output = run_object.run(cpn_input)
  File "./fate/fate/python/federatedml/model_base.py", line 209, in run
    method(cpn_input)
  File "./fate/fate/python/federatedml/model_base.py", line 263, in _run
    return ComponentOutput(self.save_data(), self.export_model(), self.save_cache())
  File "./fate/fate/python/federatedml/nn/homo_nn/enter_point.py", line 199, in export_model
    return _version_0.client_export_model(self=self)
  File "./fate/fate/python/federatedml/nn/homo_nn/_version_0.py", line 249, in client_export_model
    return _build_model_dict(meta=client_get_meta(self), param=client_get_param(self))
  File "./fate/fate/python/federatedml/nn/homo_nn/_version_0.py", line 278, in client_get_param
    param_pb.saved_model_bytes = self.nn_model.export_model()
AttributeError: 'NoneType' object has no attribute 'export_model'
[INFO] [2022-07-12 00:32:15,060] [202207120030337870380] [26410:139637714597696] - [task_base_worker.report_task_info_to_driver] [line:124]: report TaskExecutor 202207120030337870380_homo_nn_0 0 guest 9999 to driver:
{'job_id': '202207120030337870380', 'component_name': 'homo_nn_0', 'task_id': '202207120030337870380_homo_nn_0', 'task_version': '0', 'role': 'guest', 'party_id': '9999', 'run_ip': 'xxx.xxx', 'run_pid': 26410, 'party_status': 'failed', 'src_role': '', 'src_party_id': '', 'end_time': 1657585935060, 'elapsed': 8410}
[INFO] [2022-07-12 00:32:15,061] [202207120030337870380] [26410:139637714597696] - [control_client.report_task] [line:41]: request update job 202207120030337870380 task 202207120030337870380_homo_nn_0 0 on guest 9999
[INFO] [2022-07-12 00:32:15,182] [202207120030337870380] [26410:139637714597696] - [task_executor._run_] [line:254]: finish homo_nn_0 202207120030337870380_homo_nn_0 0 on guest 9999 with failed
[INFO] [2022-07-12 00:32:15,182] [202207120030337870380] [26410:139637714597696] - [base_worker.run] [line:122]: worker TaskExecutor, process role: ProcessRole.WORKER, pid: 26410, elapsed: 9383 ms

To Reproduce Steps to reproduce the behavior:

1. based on example homo_nn config
2. add cv config in example homo_nn config
3. submit job
4. See error

Expected behavior job finishes successfully

Screenshots If applicable, add screenshots to help explain your problem.

Env (please complete the following information):

• OS: centos7
• Version: fate1.7.2 in k8s

Additional context Add any other context about the problem here.