Huvu/tiktoken tokenizer update
<!-- pylint-output-strict-mode-false -->

beep boop 🤖: 🙏 The following files have warnings. In case you are familiar with these, please try helping us to improve the code base.

---

Your code was analyzed with PyLint. The following annotations have been identified:

```
************* Module nemo.collections.common.tokenizers.tiktoken_tokenizer
nemo/collections/common/tokenizers/tiktoken_tokenizer.py:65:0: C0301: Line too long (270/119) (line-too-long)
nemo/collections/common/tokenizers/tiktoken_tokenizer.py:143:4: C0116: Missing function or method docstring (missing-function-docstring)
nemo/collections/common/tokenizers/tiktoken_tokenizer.py:152:4: C0116: Missing function or method docstring (missing-function-docstring)
nemo/collections/common/tokenizers/tiktoken_tokenizer.py:187:4: C0116: Missing function or method docstring (missing-function-docstring)
nemo/collections/common/tokenizers/tiktoken_tokenizer.py:191:4: C0116: Missing function or method docstring (missing-function-docstring)
nemo/collections/common/tokenizers/tiktoken_tokenizer.py:195:4: C0116: Missing function or method docstring (missing-function-docstring)
nemo/collections/common/tokenizers/tiktoken_tokenizer.py:199:4: C0116: Missing function or method docstring (missing-function-docstring)
nemo/collections/common/tokenizers/tiktoken_tokenizer.py:203:4: C0116: Missing function or method docstring (missing-function-docstring)
nemo/collections/common/tokenizers/tiktoken_tokenizer.py:207:4: C0116: Missing function or method docstring (missing-function-docstring)
nemo/collections/common/tokenizers/tiktoken_tokenizer.py:211:4: C0116: Missing function or method docstring (missing-function-docstring)
nemo/collections/common/tokenizers/tiktoken_tokenizer.py:215:4: C0116: Missing function or method docstring (missing-function-docstring)
nemo/collections/common/tokenizers/tiktoken_tokenizer.py:229:4: C0116: Missing function or method docstring (missing-function-docstring)
nemo/collections/common/tokenizers/tiktoken_tokenizer.py:233:4: C0116: Missing function or method docstring (missing-function-docstring)
nemo/collections/common/tokenizers/tiktoken_tokenizer.py:237:4: C0116: Missing function or method docstring (missing-function-docstring)
nemo/collections/common/tokenizers/tiktoken_tokenizer.py:241:4: C0116: Missing function or method docstring (missing-function-docstring)
************* Module nemo.collections.nlp.modules.common.tokenizer_utils
nemo/collections/nlp/modules/common/tokenizer_utils.py:73:0: C0301: Line too long (199/119) (line-too-long)
nemo/collections/nlp/modules/common/tokenizer_utils.py:96:0: C0301: Line too long (149/119) (line-too-long)
nemo/collections/nlp/modules/common/tokenizer_utils.py:131:0: C0301: Line too long (146/119) (line-too-long)
nemo/collections/nlp/modules/common/tokenizer_utils.py:225:0: C0301: Line too long (146/119) (line-too-long)
nemo/collections/nlp/modules/common/tokenizer_utils.py:42:0: C0115: Missing class docstring (missing-class-docstring)
************* Module scripts.llm.t5_generate
scripts/llm/t5_generate.py:15:0: C0301: Line too long (128/119) (line-too-long)
scripts/llm/t5_generate.py:26:0: C0116: Missing function or method docstring (missing-function-docstring)

-----------------------------------
Your code has been rated at 9.20/10
```

---

Thank you for improving NeMo's documentation! [🤖]: Hi @huvunvidia 👋,

We wanted to let you know that a [CICD pipeline](https://github.com/NVIDIA/NeMo/actions/runs/12194417195) for this PR just finished successfully

So it might be time to merge this PR or get some approvals

I'm just a bot so I'll leave it you what to do next.

//cc @pablo-garay @ko3n1g
# What does this PR do ?

Updating tiktoken tokenizer to support more special tokens (e.g., <pad>, <cls>, or <extra_id_0>, <extra_id_1>,...) which are used by models such as T5.

**Collection**: [Note which collection this PR will affect]

# Changelog 
- Add specific line by line info of high level changes in this PR.

# Usage
* You can potentially add a usage example below

```python
# Add a code snippet demonstrating how to use this 
```

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# Before your PR is "Ready for review"
**Pre checks**:
- [ ] Make sure you read and followed [Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md)
- [ ] Did you write any new necessary tests?
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- [ ] Does the PR affect components that are optional to install? (Ex: Numba, Pynini, Apex etc)
  - [ ] Reviewer: Does the PR have correct import guards for all optional libraries?
  
**PR Type**:
- [ ] New Feature
- [ ] Bugfix
- [ ] Documentation

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# Additional Information
* Related to # (issue)
