
As mentioned in RFC https://github.com/vllm-project/vllm/issues/12254, this PR achieves the task: combine allocate_slots and append_slots. There should be no functionality change, except that in decode, also raise exception when num_tokens is zero (like prefill), and change the unit test case accordingly. @comaniac @rickyyx @WoosukKwon @youkaichao @heheda12345 @simon-mo --------- Signed-off-by: Shawn Du <shawnd200@outlook.com>