code: def save_server_model(self, model_path):
      self.fl_client.nn_stub.save_server_model(
          SaveModelRequest(model_path=model_path, backend="pt")
      )
  
imports: import logging
  from numpy import ndarray
  import torch
  from torch import nn
  from bigdl.ppml.fl.nn.fl_client import FLClient
  from torch.utils.data import DataLoader
  from bigdl.dllib.utils.log4Error import invalidInputError, invalidOperationError
  from bigdl.ppml.fl.nn.nn_client import NNClient
  from bigdl.ppml.fl.nn.utils import file_chunk_generate, tensor_map_to_ndarray_map
  import os
  import tempfile
  from nn_service_pb2 import LoadModelRequest, SaveModelRequest
label_desc: Use image node_id 2 for calling the PytorchEstimator obj's underlying member method code with example usage: obj.save_server_model(model_path) without return types