Removing external code when models are upstreamed into Transformers

As of writing, we keep Ernie 4.5/4.5 MoE code in `external` and tell the engine to use this code in `utils/register_models.py`. This pattern was introduced in #185. However, as of Transformers 4.54.0, these models now exist upstream in Transformers, resulting in an error when we try to overwrite the registration. #209 solves this by setting `exist_ok=True` for upstreamed models, but the need for this workaround should really serve as an indication that external code *should be removed*.

Start with the Ernie 4.5/4.5 MoE code under `external` and its registration in `utils/register_models.py`, then trace the pattern introduced by #185. Done means the upstreamed model code and registration are removed, so the workaround from #209 is no longer needed.

Written by the indexing model from the issue text.