Rectify code of the LayoutLM series models and adjust it to amp_level mode

[Bourn3z]

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Motivation

之前由于amp功能bug，LayoutLM系列模型全部通过手动插入cast算子的方式，实现混合精度训练加速。现经过验证问题已解决，因此整改LayoutLM系列模型代码，调整yaml中控制混合精度参数的方式。

~~use_float16: True~~

Test Plan

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Related Issues and PRs

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