Cannot tokenize byte sequences that are not valid UTF-8 due to design flaw

Hello,

The BPE algorithm is capable of tokenizing any byte sequence, and LLMs generally accept any sequence of tokens and use token dictionaries that can successfully represent any byte sequence, but the encode method in bpe-openai accepts a type that has to be valid UTF-8. So there are lots of byte sequences, many of which are only 1 byte long, which you cannot tokenize using this library.

Start by inspecting the bpe-openai encode method and the input type it accepts. Determine how the library should expose arbitrary byte sequences, then add coverage for invalid UTF-8 inputs and verify that all byte sequences can be tokenized.

Written by the indexing model from the issue text.