This PR closes#27781
# Problem
The current implementation of `NLTKTextSplitter` is using
`sent_tokenize`. However, this `sent_tokenize` doesn't handle chars
between 2 tokenized sentences... hence, this behavior throws errors when
we are using `add_start_index=True`, as described in issue #27781. In
particular:
```python
from nltk.tokenize import sent_tokenize
output1 = sent_tokenize("Innovation drives our success. Collaboration fosters creative solutions. Efficiency enhances data management.", language="english")
print(output1)
output2 = sent_tokenize("Innovation drives our success. Collaboration fosters creative solutions. Efficiency enhances data management.", language="english")
print(output2)
>>> ['Innovation drives our success.', 'Collaboration fosters creative solutions.', 'Efficiency enhances data management.']
>>> ['Innovation drives our success.', 'Collaboration fosters creative solutions.', 'Efficiency enhances data management.']
```
# Solution
With this new `use_span_tokenize` parameter, we can use NLTK to create
sentences (with `span_tokenize`), but also add extra chars to be sure
that we still can map the chunks to the original text.
---------
Co-authored-by: Erick Friis <erick@langchain.dev>
Co-authored-by: Erick Friis <erickfriis@gmail.com>
