Created by: Aurelius84

while usingwhile_loop in static graph, if python mutable object like dict/list/set is involved in loop_vars, the object (or var) shall not add or delete element in body function.

In this PR, we add assert_same_structure to check output_vars.

If a list var add a element in body function, it will raise ValueError as follows:

ValueError: body in while_loop should return the same arity (length and structure) and types as loop_vars: The two structures don't have the same number of elements.

First structure (3 elements): (name: "fill_constant_0.tmp_0"
type {
  type: LOD_TENSOR
  lod_tensor {
    tensor {
      data_type: INT64
      dims: 1
      ......

Second structure (2 elements): [name: "fill_constant_0.tmp_0"
type {
  type: LOD_TENSOR
  lod_tensor {
    tensor {
      data_type: INT64
      dims: 1
      ......

Assertion failed

Process finished with exit code 1