convert : import bytes_to_unicode from convert_slow_tokenizer (#26217)
bytes_to_unicode was removed from transformers.models.gpt2.tokenization_gpt2
in huggingface/transformers#40936, but it had already been copied into
transformers.convert_slow_tokenizer in huggingface/transformers#30334
(transformers 4.54.1), so import it directly from there.
Applies the same fix to chatglm.py.