[`LlamaConfig`] Nit: pad token should be None by default
# What does this PR do?
Does not set the padding token as `0` is for the `unk_token`. Though Llama has bytefallback support, meaning most of the time the unkown token will not be form the input, it can generate the `<unk>` token (probably very rarely). The pad token is used in the embedding and the unk embedding is not None.

This should warn user that are doing `SequenceClassification` that the padding token is not set.
_The documentation is not available anymore as the PR was closed or merged._