Fix handling of Sequence post-processors in train_new_from_iterator
Author: taidopurason
Pull request: yes

# 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": "
",
            "type_id": 0
          }
        },
        {
          "Sequence": {
            "id": "A",
            "type_id": 0
          }
        }
      ],
      "pair": [
        {
          "SpecialToken": {
            "id": "
",
            "type_id": 0
          }
        },
        {
          "Sequence": {
            "id": "A",
            "type_id": 0
          }
        },
        {
          "SpecialToken": {
            "id": "
",
            "type_id": 1
          }
        },
        {
          "Sequence": {
            "id": "B",
            "type_id": 1
          }
        }
      ],
      "special_tokens": {
        "
": {
          "id": "
",
          "ids": [
            128000
          ],
          "tokens": [
            "
"
          ]
        }
      }
    }
  ]
}
```
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).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#create-a-pull-request),
      Pull Request section?
- [x] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?

- tokenizers: @ArthurZucker



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