Add support for non-rust implemented tokenization for `__getitem__` method.
# Overview
This PR is going to add a support for the usage scenario of "getting a slice from the batch-tokenized sequences".

Without this PR, it seems to raise KeyError with the following message KeyError: 'Indexing with integers (to access backend Encoding for a given batch index) is not available when using Python based tokenizers'

P.S. The above scenario could be reproduced by using some models new uploaded but not support to Rust-implemented tokenization, such as fnlp/moss-moon-003-sft. Also we can run the following examplar script for reproducing this issue:

```python
# test script `/home/workspace/test.py` for this PR. 
from transformers import AutoTokenizer

tok = AutoTokenizer.from_pretrained("fnlp/moss-moon-003-sft", trust_remote_code=True)
tok.add_special_tokens({"pad_token": "[PAD]"})

texts = ["Today is a good day!", "It's a good idea!", "How's going?"]
batch_tok = tok(texts, padding=True)
print(batch_tok[0:3])  # report `KeyError` here
```
# Error Message
```txt
Traceback (most recent call last):
  File "/home/workspace/test.py", line 8, in <module>
    print(batch_tok[0:3])  # report `KeyError` here
  File "/home/app/anaconda3/envs/test/lib/python3.9/site-packages/transformers/tokenization_utils_base.py", line 242, in __getitem__
    raise KeyError(
KeyError: 'Indexing with integers (to access backend Encoding for a given batch index) is not available when using Python based tokenizers'
```

All in all, I think it seems useful to implement __getitem__ method behind it in Python side :)

Note that this PR is associative with the previous closed one. 
https://github.com/huggingface/transformers/pull/23645
It seems that failed in workflow due to `Read Timeout` on T5-relevant testing.
How can I rerun for this?

![图片](https://github.com/huggingface/transformers/assets/54089835/f9922d94-440f-42d6-92b7-69f38e604d81)
![图片](https://github.com/huggingface/transformers/assets/54089835/4a1eec18-f2cf-4509-93aa-1c254286fe2e)
<|||||>We can re-run that for you 😉 
<|||||>_The documentation is not available anymore as the PR was closed or merged._<|||||>Request for review :)<|||||>@jacklanda Could you update the error message as requested by @ArthurZucker? <|||||>> @jacklanda Could you update the error message as requested by @ArthurZucker?

@amyeroberts Have updated the mentioned error messages by @ArthurZucker 
Thanks.<|||||>Ask for final review :)