docstr: 测试单个Jittor Var的转换
docstr_tok: ["\u6d4b\u8bd5\u5355\u4e2aJittor","Var\u7684\u8f6c\u6362"]
code: def test_var_transfer(self):
      """
      测试单个Jittor Var的转换
      """
  
      jittor_var = jittor.rand((3, 4, 5))
      res = jittor2torch(jittor_var)
      if jittor.flags.use_cuda:
          self.check_torch_tensor(res, "cuda:0", True)
      else:
          self.check_torch_tensor(res, "cpu", True)
  
      res = jittor2torch(jittor_var, device="cuda:2", no_gradient=None)
      self.check_torch_tensor(res, "cuda:2", True)
  
      res = jittor2torch(jittor_var, device="cuda:2", no_gradient=True)
      self.check_torch_tensor(res, "cuda:2", False)
  
      res = jittor2torch(jittor_var, device="cuda:2", no_gradient=False)
      self.check_torch_tensor(res, "cuda:2", True)
  
imports: import pytest
  from fastNLP.envs.imports import _NEED_IMPORT_JITTOR, _NEED_IMPORT_PADDLE, _NEED_IMPORT_TORCH
  from fastNLP.modules.mix_modules.utils import paddle2torch, torch2paddle, jittor2torch, torch2jittor
label_desc: Use image node_id 2 for calling the TestJittor2Torch obj's underlying member method code with example usage: obj.test_var_transfer() without return types