[Misc][Platform] Move use allgather to platform

[MengqingCao]

Mainly changes

1. Move use_all_gather to platform so that the oot devices could override it instead of modifying in vLLM
2. Add check of sampling_metadata.seq_groups before applying logits processor, as when using all-gather, the logits is not none, but sampling_metadata.seq_groups is none, so _apply_logits_processors will raise an unexpected error. We add this check to avoid step into _apply_logits_processors when using all-gather

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[MengqingCao]

@youkaichao could you help review this? thx!

[youkaichao]

sounds good

[MengqingCao]

CI failed due to network issue