Optimize topk's performance when k is small and input_width is large

[carryyu]

PR types

Performance optimization

PR changes

OPs

Describe

Optimize topk's performance when k is small and input_width is large

dtype：FP32，循环测试10w次，取平均值。 优化后在k值较小且input_width较大时速度提升3-4倍。

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