Add SeizureTransformer, the 2025 SzCORE seizure detection winner #1199

Hi @bruAristimunha! Can I work on adding SeizureTransformer to braindecode? It won the 2025 SzCORE seizure detection challenge, and it's first on the sheet in #828 with no one listed for it yet.

• Paper: https://arxiv.org/abs/2504.00336
• Code: https://github.com/keruiwu/SeizureTransformer (MIT)

I already ran the released model on braindecode's Siena dataset and got the same scores as the leaderboard, so there's a clear target to check the port against. Unless someone from @danjjl's group has already started on it, I'd be happy to take it.