Update PaliGemmaWithExpertModel to handle multiple transformer versions

[skalade]

What this does

Tried training pi0 policy and got AttributeError: 'GemmaModel' object has no attribute 'model' in paligemma_with_expert.py.

Looks like this similar to issue addressed in this merged PR #1247. I copied the same logic for embed_language_tokens and paligemma.language_model, now training script runs fine with pi0.

How it was tested

Ran training script with policy.type=pi0

How to checkout & try? (for the reviewer)

python lerobot/scripts/train.py \
    --policy.type=pi0 \
    --dataset.repo_id=lerobot/pusht \
    --dataset.video_backend=pyav \
    --policy.device=cuda

[fracapuano]

Hey @skalade 👋 thank you so much for your PR! we recently bumped the minimal dependency for transformers (#1298) so not to have this issue 😊 Hence, I am afraid we'll close this PR, but thank you again 🤗 ⭐