`Qwen3Tokenizer` fix for Qwen3 Base models and generation mismatch with HF (#828)

* prevent `self.apply_chat_template` being applied for base Qwen models

* - added no chat template comparison in `test_chat_wrap_and_equivalence`
- removed duplicate comparison

* Revert "- added no chat template comparison in `test_chat_wrap_and_equivalence`"

This reverts commit 3a5ee8cfa19aa7e4874cd5f35171098be760b05f.

* Revert "prevent `self.apply_chat_template` being applied for base Qwen models"

This reverts commit df504397a8957886c6d6d808615545e37ceffcad.

* copied `download_file` in `utils` from https://github.com/rasbt/reasoning-from-scratch/blob/main/reasoning_from_scratch/utils.py

* added copy of test `def test_tokenizer_equivalence()` from `reasoning-from-scratch` in `test_qwen3.py`

* removed duplicate code fragment in`test_chat_wrap_and_equivalence`

* use apply_chat_template

* add toggle for instruct model

* Update tokenizer usage

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Co-authored-by: rasbt <mail@sebastianraschka.com>