Add multi prompt support for beam search #510

Job	Run time
cpp-beam_search_causal_lm-Qwen1_5-7B-Chat	5m 2s
cpp-beam_search_causal_lm-Phi-2	4m 6s
cpp-beam_search_causal_lm-Mistral-7B	5m 54s
cpp-beam_search_causal_lm-notus-7b-v1	4m 48s
cpp-beam_search_causal_lm-Qwen-7B-Chat	5m 28s
cpp-beam_search_causal_lm-ubuntu	6m 16s
cpp-beam_search_causal_lm-windows	13m 11s
cpp-greedy_causal_lm-ubuntu	4m 22s
cpp-Phi-1_5	3m 54s
cpp-speculative_decoding_lm-ubuntu	10m 9s
	1h 3m 10s

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