Fix handling of Sequence post-processors in train_new_from_iterator

[taidopurason]

What does this PR do?

This PR fixes an issue where the post-processor special token IDs are not correctly updated when training a new tokenizer using train_new_from_iterator of a tokenizer with a Sequence post-processor. Instead, the special token IDs are copied directly from the original tokenizer.

For example, this affects training a new tokenizer from Llama-3 tokenizers, as reported in #33998 and #30752.

Running the following code:

from transformers import AutoTokenizer
from datasets import load_dataset
import json
from itertools import islice

tokenizer = AutoTokenizer.from_pretrained("meta-llama/Meta-Llama-3.1-8B")
ds = load_dataset("wikimedia/wikipedia", "20231101.et", streaming=True, split="train")

new_tokenizer = tokenizer.train_new_from_iterator([x["text"] for x in islice(ds, 100)], 1000)

print(f"bos_token_id={new_tokenizer.bos_token_id}")
print(f"'Hello world!' tokenized as {new_tokenizer('Hello world!')['input_ids']}")
print(json.dumps(json.loads(new_tokenizer._tokenizer.to_str())['post_processor'], indent=2))

the output is:

bos_token_id=0
'Hello world!' tokenized as [128000, 294, 569, 727, 399, 338, 541, 327, 319, 256]
{
  "type": "Sequence",
  "processors": [
    {
      "type": "ByteLevel",
      "add_prefix_space": true,
      "trim_offsets": false,
      "use_regex": true
    },
    {
      "type": "TemplateProcessing",
      "single": [
        {
          "SpecialToken": {
            "id": "<|begin_of_text|>",
            "type_id": 0
          }
        },
        {
          "Sequence": {
            "id": "A",
            "type_id": 0
          }
        }
      ],
      "pair": [
        {
          "SpecialToken": {
            "id": "<|begin_of_text|>",
            "type_id": 0
          }
        },
        {
          "Sequence": {
            "id": "A",
            "type_id": 0
          }
        },
        {
          "SpecialToken": {
            "id": "<|begin_of_text|>",
            "type_id": 1
          }
        },
        {
          "Sequence": {
            "id": "B",
            "type_id": 1
          }
        }
      ],
      "special_tokens": {
        "<|begin_of_text|>": {
          "id": "<|begin_of_text|>",
          "ids": [
            128000
          ],
          "tokens": [
            "<|begin_of_text|>"
          ]
        }
      }
    }
  ]
}

As shown, the new tokenizer prepends an incorrect bos_token_id (128000 instead of 0)

Fixes #33998 #30752

I welcome feedback and suggestions on this fix.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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  here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review?

• tokenizers: @ArthurZucker

[LysandreJik]

Thanks for the issue! Also cc @itazap for knowledge (but I know your bandwidth is very limited right now)

[itazap]

Thanks for working on this!! 👍
Should this also be updated to set to _post_processors?

Also, would be great to add a test for this so that we can catch this in the future! Let me know if you would like to contribute the test or I can add it! 🤗

[taidopurason]

Thanks for the feedback! The reason for using _post_processor on the final block is to ensure that if Roberta or BERT processors are ever part of the Sequential postprocessor, their cls and sep token IDs will be correctly updated. I can revert this if you think it’s unnecessary.

Including a unit test is a great idea - I’ll start working on it.

[taidopurason]

Added a test case for the Sequence post-processor. @itazap, let me know what you think. Should anything else be added or changed?