[Feature]: Adding xLSTM

[HallerPatrick]

Problem statement

As part of this issue following improvements will be added:

• Expand LanguageModel and LanguageModelTrainer that it can use xLSTM (sLSTM and mLSTM)
• Allow subtoken tokenization fo LM training

Solution

The new officially xLSTM library just dropped. So its probably wise to use it instead of implementing it from scratch (though we have to check of dependency issues)

Additional Context

No response