Making TF TransfoXL model compliant with AMP
# What does this PR do?

This PR makes the TF TransfoXL model compliant with AMP. All the slow tests are passing as well for these models.

These two models cannot be XLA compliant for now, as it seems that tf.where cannot be used in XLA if the x and y parameters are None. See the _get_global_attn_indices method which has this case. I have opened [an issue](https://github.com/tensorflow/tensorflow/issues/47211) on the TF repo in order to ask if it is an expected behavior or a bug.
