remove superfluous 0 from lod

[mzchtx]

In some cases, all output sequences were empty, therefore, the LOD should be [[0]] rather than [[0, 0]].

https://github.com/PaddlePaddle/Paddle/blob/develop/python/paddle/fluid/layers/nn.py#L2665

[wanghaoshuang]

对于n个空sequence, LOD应该是[[0] * n], 我文档里边写错了。
如果方便的话，在这个PR里帮我改下文档？

[paddle-bot-old]

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