disable kernel promotion for amp traning #1085
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背景：过去框架AMP在O2模式下，当OP支持低精度就会选择低精度的kernel，但这样的策略出现精度问题的风险较高。为保障训练精度，框架在2.5版本对AMP 策略进行了调整，即在O2模式下，仅当Op所有输入为低精度时才会选择低精度kernel，否则则采用FP32 Kernel（即promote的策略），因此可能会引起部分模型出现性能下降。
目前可以通过给auto_cast设置use_promote=False参数来回退到旧版本的O2策略，本PR给套件添加该参数的设置功能，当前套件默认设置为use_promote=False，使用的是旧版本的O2策略，以解决性能下降问题。
PaddlePaddle框架动态图下默认的行为是use_promote=True，未来可以根据模型特点进行调试。
框架PR：PaddlePaddle/Paddle#53742