Llama: support for `max_position_embeddings`
# What does this PR do?

Related to #22433.

`LlamaConfig`, as opposed to other models, did not support `max_position_embeddings`. Instead, the position embedding class was hardcoded to a length of `2048`. This PR rectifies that, keeping the `2048` default.
_The documentation is not available anymore as the PR was closed or merged._