Created by: liym27

This PR supports list.append() in control flow if、for、while

Python list will be transformed into LoDTensorArray. In order to use LoDTensorArray like a python list, more support is needed. LoDTensorArray is supported in stack op: https://github.com/PaddlePaddle/Paddle/pull/22962

Example 1, in control flow if

• before:

 def test_list_in_if(x):
    x = fluid.dygraph.to_variable(x)
    a = []
    if x.numpy()[0] > 0:
        a.append(x)
    else:
        a.append(fluid.layers.fill_constant(shape=[1, 2], value=9, dtype=
            'int64'))
    return a

• after:

 def test_list_in_if(x):
    x = fluid.layers.assign(x)
    a = fluid.layers.create_array(dtype='float32')

    def true_fn_0(a, x):
        fluid.layers.array_write(x=x, i=fluid.layers.array_length(a), array=a)

    def false_fn_0(a):
        fluid.layers.array_write(x=fluid.layers.fill_constant(shape=[1, 2],
            value=9, dtype='int64'), i=fluid.layers.array_length(a), array=a)
    fluid.layers.cond(x[0] > 0, lambda : true_fn_0(a, x), lambda :
        false_fn_0(a))
    return a

Example 2, in control flow while

• before:

 def test_list_in_while_loop(x, iter_num):
    x = fluid.dygraph.to_variable(x)
    iter_num = fluid.layers.fill_constant(shape=[1], value=iter_num, dtype=
        'int32')
    a = []
    i = 0
    while i < iter_num.numpy()[0]:
        a.append(x)
        i += 1
    return a

• after:

def test_list_in_while_loop(x, iter_num):
    x = fluid.layers.assign(x)
    iter_num = fluid.layers.fill_constant(shape=[1], value=iter_num, dtype=
        'int32')
    a = fluid.layers.create_array(dtype='float32')
    i = 0
    a = fluid.dygraph.dygraph_to_static.variable_trans_func.to_static_variable(
        a)
    x = fluid.dygraph.dygraph_to_static.variable_trans_func.to_static_variable(
        x)
    iter_num = (fluid.dygraph.dygraph_to_static.variable_trans_func.
        to_static_variable(iter_num))
    i = fluid.dygraph.dygraph_to_static.variable_trans_func.to_static_variable(
        i)

    def while_condition_0(a, x, iter_num, i):
        return i < iter_num[0]

    def while_body_0(a, x, iter_num, i):
        fluid.layers.array_write(x=x, i=fluid.layers.array_length(a), array=a)
        i += 1
        return a, x, iter_num, i
    a, x, iter_num, i = fluid.layers.while_loop(while_condition_0,
        while_body_0, [a, x, iter_num, i])
    return a