[Frontend][PaddlePaddle] Add topk op and Fix bug, when the output is a dimension, it should not be squeezed

[woobinw]

There are some operation in PaddlePaddle that output a tensor which shape is [1], meanwhile, the slice op have decrease_axis attr, but it should not be squeezed.

@heliqi
@jiangjiajun
cc @junrushao

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