Rework the AMP for TF XLNet
# What does this PR do?

This PR reworks the AMP of XLNet to remove some useless casts for better and less confusing AMP compliancy.
Yes, `bfloat16` is only for TPU. Hence, we cannot really test it elsewhere than inside a TPU context. I have added the `bfloat16` condition only if XLNet is run on TPU because we were handling a specific case when the model is run under AMP.