Sberbank 🤗 repository name changed & skip_special_tokens removal

[valeriylo]

1. On project set-up, when trying to load models from sberbank huggingface repos, transformer lib throws unknown repo name error. It happens because of sberbank repo name has been changed from sberbank-ai to ai-forever. For future-proof it's preferable to store model checkpoints in author's own 🤗 repository.
2. When using add-sent transformation, which has gtp-3 model by default, code throws error, that indicates skip_special_tokens is not in the parameters list of gtp-3 model. So it is suggested to be removed or for-loop to be refactored