Making TF BART-like models XLA and AMP compliant
# What does this PR do?

This PR makes the TF BART-like models compliant with AMP and XLA. The main issue for XLA was all the asserts, XLA is not compliant with them (see the [TF doc](https://www.tensorflow.org/xla/known_issues)), so I had to disable them if the model is run with another mode than eager.

TF Marian and Pegasus have still their XLA test locked because they are not working for XLA_GPU. I need to investigate more in order to better understand why. My first guess is because of the `TFXSinusoidalPositionalEmbedding` class.
I succeed to fix Marian and Pegasus, and my first guess was the good one. I basically reworked a bit how the embedding was created, and now it works in XLA_GPU. Of course, all the corresponding slow tests are passing, and the weights are properly loaded.