[Bugfix][Core] add seq_id_to_seq_group clearing to avoid memory leak when s…

[zhangyuygss]

Fix CPU memory leak when generate n completions per-prompt.

Related issue:
FIX #12973

When set n>1, vllm uses ParallelSampleSequenceGroup to manage and put requests into engine.
In ParallelSampleSequenceGroup, it keeps adding id: processed request pair to engine.seq_id_to_seq_group:

class ParallelSampleSequenceGroup(SequenceGroupBase):

    @staticmethod
    def add_request(request_id: str, engine, params, **kwargs):
        original_params = params
        group = ParallelSampleSequenceGroup(request_id)
        seqs = []
        for i in range(original_params.n):
            request_id_i = f"{request_id}_parallel_sample_{i}"
            group.seq_id_to_index[request_id_i] = i
            params = copy.deepcopy(original_params)
            params.n = 1
            if params.seed is not None:
                params.seed += i
            seq_group = engine._add_processed_request(
                request_id_i,
                params=params,
                **kwargs,
            )  # type: ignore
            assert seq_group is not None
            engine.seq_id_to_seq_group[request_id_i] = group
            group.to_be_finished[request_id_i] = seq_group
            seqs.append(seq_group.seqs[0])

seq_id_to_seq_group is never released during inference.

Adding clearing codes to free the memory.

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[youkaichao]

I thought #14326 should fix the problem. didn't know there's another place of memory leak.

[DarkLight1337]

Tests pass so this should be good to go, thanks