class_docstr: 1） 如果DataSet使用dict初始化，那么在add_field中会构造FieldArray：
          1.1) 二维list  DataSet({"x": [[1, 2], [3, 4]]})
          1.2) 二维array  DataSet({"x": np.array([[1, 2], [3, 4]])})
          1.3) 三维list  DataSet({"x": [[[1, 2], [3, 4]], [[1, 2], [3, 4]]]})
      2） 如果DataSet使用list of Instance 初始化,那么在append中会先对第一个样本初始化FieldArray；
      然后后面的样本使用FieldArray.append进行添加。
          2.1) 一维list DataSet([Instance(x=[1, 2, 3, 4])])
          2.2) 一维array DataSet([Instance(x=np.array([1, 2, 3, 4]))])
          2.3) 二维list  DataSet([Instance(x=[[1, 2], [3, 4]])])
          2.4) 二维array  DataSet([Instance(x=np.array([[1, 2], [3, 4]]))])
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code: def test_init_v7(self):
      # list of array
      fa = FieldArray(
          "x", [np.array([[1, 2], [3, 4]]), np.array([[1, 2], [3, 4]])]
      )
  
imports: import os
  import pytest
  import numpy
  from fastNLP.core.dataset import DataSet, FieldArray, Instance, ApplyResultException
  from fastNLP import logger
label_desc: Use image node_id 7 for calling the TestFieldArrayInit obj's underlying member method code with example usage: obj.test_init_v7() without return types