Fix ESM LM head test
The original ESM-2 checkpoints had a bug that meant the LM head bias was not saved correctly. Now this has been fixed, we need to update our LM test as well.

cc @ydshieh
_The documentation is not available anymore as the PR was closed or merged._<|||||>@ydshieh Unfortunately I deleted and remade the repos, so there's no commit to point at! I'll try to do that in future though.