BOS tokens not properly added in some circumstances

[jquesnelle]

When using DocumentTokenizer if an eos_token is specified the tokenizer post processor is replaced with one that appends and EOS. However, this has the effect of NOT placing a BOS token at the beginning of the sequence.

See here: https://github.com/huggingface/datatrove/blob/main/src/datatrove/utils/tokenization.py#L55

This can be reproduced by tokenizing with a tokenizer like Llama 3 and looking at the raw token values

[guipenedo]

That's a good point, would you be willing to make a PR?