Add multi prompt support for beam search #512

Job	Run time
cpp-beam_search_causal_lm-windows	10m 14s
cpp-beam_search_causal_lm-Mistral-7B	5m 54s
cpp-beam_search_causal_lm-notus-7b-v1	5m 9s
cpp-beam_search_causal_lm-Qwen1_5-7B-Chat	5m 23s
cpp-beam_search_causal_lm-Phi-2	4m 20s
cpp-beam_search_causal_lm-Qwen-7B-Chat	8m 28s
cpp-beam_search_causal_lm-ubuntu	7m 45s
cpp-greedy_causal_lm-ubuntu	5m 21s
cpp-speculative_decoding_lm-ubuntu	8m 52s
cpp-Phi-1_5	4m 10s
	1h 5m 36s

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