fix sequential bug about key error

[wanghuancoder]

PR types

Bug fixes

PR changes

Others

Describe

#39220 中反馈了Sequential的一个bug：

import paddle

layers = paddle.nn.Sequential()
for i in range(10):
    layers.add_sublayer('block'+str(i), paddle.nn.Linear(10, 10))
for layer in layers:
    print(layer)

会报KeyError
本PR修复了该bug

[paddle-bot-old]

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[JiabinYang]

LGTM