Tweak ESM tokenizer for Nucleotide Transformer
Nucleotide Transformer is a model that takes DNA inputs. It uses the same model architecture as the protein model ESM, but in addition to a different vocabulary it tokenizes inputs without a `<sep>` or `<eos>` token at the end. This PR makes a small tweak to the tokenization code for ESM, so that it doesn't try to add `self.eos_token_id` to sequences when the tokenizer does not have an `eos_token` set. With this change, we can fully support Nucleotide Transformer as an ESM checkpoint.
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