Fix Tokenization + misc fixes

[hynky1999]

• Fixes the info metric_aggregated default constructor. This was causing errror when one used multiple different metrics for one task (e.g pmi and token norm)
• Fixes pair tokenization:
  1. In case on pairwise tokenization the continuation no longer has added special tokens + it's simplified
  2. The non-pairwise tokenization now uses last token as continuation in case the last token gets merged
• Add pair tokenization to VLLM
• Changes multiprocessing to multiprocess which allow to use lambda functions and partial applications in task configs (before this would fail when dataset loading process was > 1 and multiprocessing was used)