You can implement a Byte-Level Tokenizer from scratch using SentencePiece by training a model with the --byte_fallback option to ensure byte-level granularity.
Here is the code snippet below:

In the above code we are using the following key points:
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SentencePieceTrainer for training a byte-level tokenizer.
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byte_fallback=True ensures that unseen characters are broken down into bytes.
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Loading and encoding/decoding with SentencePieceProcessor for practical use.
Hence, this method builds a tokenizer that can handle any input robustly at the byte level, making it well-suited for diverse LLM tasks.