BUG: TokenizerGroup doesn't behave like a PreTrainedTokenizer

[saattrupdan]

The new TokenizerGroup class that LLM.llm_engine.tokenizer has now become, doesn't behave like a tokenizer.

For instance, the LLM.set_tokenizer method sets the tokenizer attribute as a PreTrainedTokenizer, not a TokenizerGroup.

Also, tools like lm-format-enforcer assumes that the tokenizer attribute is indeed a tokenizer, causing it to now give AttributeErrors.

It seems like either the LLM.llm_engine.tokenizer should revert back to being a PreTrainedTokenizer, or otherwise at least have properties and methods which call the corresponding properties and methods of the underlying PreTrainedTokenizer.

[zhuohan123]

Should be fixed by #2741.