[BugFix] Fix recovery logic for sequence group

[WoosukKwon]

When a sequence group has N sequences, N - 1 of which are terminated, the remaining sequence can be recovered from preemption using the re-computation mechanism. However, currently the scheduler assumes that every sequence in a waiting sequence group is in the waiting state. This PR fixes this error.

An example test case:

from vllm import LLM, SamplingParams

# Configured for A100-80GB GPU.
llm = LLM("meta-llama/Llama-2-13b-hf", gpu_memory_utilization=0.5, swap_space=20)

num_prompts = 1000
prompt_len = 300
llm.generate(
    prompt_token_ids=[[0] * prompt_len for _ in range(num_prompts)],
    sampling_params=SamplingParams(max_tokens=100, n=2))

[WoosukKwon]

@zhuohan123 This PR is ready for review. Please take a look at it!

[zhuohan123]

LGTM! Thanks for the fix!