code: def _build_onnx(
      self, x, dirname=None, thread_num=None, sess_options=None
  ):
      if not self.model_built:
          invalidInputError(
              False,
              "You must call fit_eval or restore first before"
              " calling onnx methods!",
          )
      try:
          import onnx
          import onnxruntime
      except:
          invalidInputError(
              False,
              "You should install onnx and onnxruntime to use onnx based method.",
          )
      if dirname is None:
          dirname = tempfile.mkdtemp(prefix="onnx_cache_")
      # code adapted from
      # https://pytorch.org/tutorials/advanced/super_resolution_with_onnxruntime.html
      torch.onnx.export(
          self.model,
          x,
          os.path.join(dirname, "cache.onnx"),
          export_params=True,
          opset_version=10,
          do_constant_folding=True,
          input_names=["input"],
          output_names=["output"],
          dynamic_axes={
              "input": {0: "batch_size"},
              "output": {0: "batch_size"},
          },
      )
      self.onnx_model = onnx.load(os.path.join(dirname, "cache.onnx"))
      onnx.checker.check_model(self.onnx_model)
      if sess_options is None:
          sess_options = onnxruntime.SessionOptions()
          if thread_num is not None:
              sess_options.intra_op_num_threads = thread_num
      self.ort_session = onnxruntime.InferenceSession(
          os.path.join(dirname, "cache.onnx"), sess_options=sess_options
      )
      self.onnx_model_built = True
  
imports: import torch
  from torch.utils.data import TensorDataset, DataLoader
  import types
  import numpy
  import math
  import pandas
  import tempfile
  import os
  import copy
  from bigdl.orca.automl.model.abstract import BaseModel, ModelBuilder
  from bigdl.orca.automl.metrics import Evaluator
  from bigdl.orca.automl.pytorch_utils import LR_NAME, DEFAULT_LR
  from bigdl.dllib.utils.log4Error import *
label_desc: Use image node_id 22 for calling the PytorchBaseModel obj's underlying member method code with example usage: obj._build_onnx(x, dirname, thread_num, sess_options) without return types