Fixing block size for Mistral-7B.

[Artyom17]

According to Mistral's paper the block size for Mistral-7B should be 8192 (ref: https://arxiv.org/pdf/2310.06825.pdf, https://huggingface.co/docs/transformers/en/model_doc/mistral). But currently it is set to the default value (2048).