use AutoTokenizer instead of LlamaTokenizer in checkpoint_converter_fsdp_hf.py

[KeitaW]

What does this PR do?

Feature/Issue validation/testing

Please describe the tests that you ran to verify your changes and relevant result summary. Provide instructions so it can be reproduced. Please also list any relevant details for your test configuration.

Since LlamaTokenizer is not compatible with Llama3 tokenizers, running checkpoint_converter_fsdp_hf.py with llama3 finetuned weights result in TypeError: not a string error (cf. https://github.com/huggingface/transformers/issues/30607). This PR is suggesting to use AutoTokenizer instead to make the script compatible with both Llama2/3.

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• [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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