Making TF Longformer-like models compliant with AMP
# What does this PR do?

This PR makes the TF Longformer-like models 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.
