code: def test_elemwise_add_grad():
      json = '{"nodes": [{"op":"null","name":".Inputs.Input1","inputs":[]},{"op":"null","name":".Inputs.Input2","inputs":[]},{"op":"elemwise_add","name":".$0","inputs":[[0,0,0],[1,0,0]]},{"op":"_copy","name":".Outputs.Output","inputs":[[2,0,0]]}],"arg_nodes":[0,1],"heads":[[3,0,0]]}'
      sym = mx.symbol.fromjson(json)
  
      ex = sym._bind(
          mx.cpu(),
          {
              ".Inputs.Input1": mx.nd.array([0.4]),
              ".Inputs.Input2": mx.nd.array([0.5]),
          },
          args_grad={
              ".Inputs.Input1": mx.ndarray.zeros((1)),
              ".Inputs.Input2": mx.ndarray.zeros((1)),
          },
          grad_req={
              ".Inputs.Input1": "null",
              ".Inputs.Input2": "write",
          },
      )
      ex.forward(is_train=True)
      print(ex.outputs)
      ex.backward(out_grads=mx.nd.array([1]))
      print(ex.grad_arrays)
  
file_path: mxnet/tests/python/unittest/test_executor.py
imports: import numpy
  import mxnet
  from mxnet.test_utils import assert_almost_equal, environment
label_desc: Use image node_id 6 for calling a global function with example usage: test_elemwise_add_grad() without return types