Created by: liym27

Before this PR, list a won't be transformed to LoDTensorArray because a is reassigned with a Tensor fluid.layers.concat(a, axis=0). However, a=[] should be transformed into a=fluid.layers.create_array(dtype="float32").

def test_list_in_for_loop_with_concat(x, iter_num):
    x = fluid.dygraph.to_variable(x)
    a = []  # Before this PR, `a=[]` is not transformed
    for i in range(iter_num):
        a.append(x)
    a = fluid.layers.concat(a, axis=0)
    return a