Fix tokenization edge case where llama output does not start with a space

[noamgat]

Created following investigation in this issue:
noamgat/lm-format-enforcer#92

See this notebook for a reproduction of the problem:
https://colab.research.google.com/drive/1Ooz11nFPk19zyJdMDx42CeesU8aWZMdI#scrollTo=oKpHw5PZ30uC

When using the model
TheBloke/tinyllama-1.1b-chat-v1.0-GGUF

In the current implementation, the token sequence [6377] decodes to {" while the token sequence [1,6377] decodes to ". This is because the LLama tokenizer doesn't add a leading space when decoding this sequence, but the llama-cpp-python code that wraps it assumes that it does.
This breaks that assumption, and only returns output[1:] instead of output when the first character is a space.

Implementation note: I made the check output[0:1] == ' ' and not output[0] == ' ' to avoid edge cases where the output is empty (maybe if the first tokens are partial unicode characters).

[noamgat]

Is it possible to review this? I think its a very straightforward fix.

[abetlen]

@noamgat yup and thank you for looking into this