Add multi prompt support for beam search #531

Job	Run time
cpp-beam_search_causal_lm-windows	13m 11s
cpp-beam_search_causal_lm-Phi-2	3m 10s
cpp-beam_search_causal_lm-Mistral-7B	5m 55s
cpp-beam_search_causal_lm-notus-7b-v1	4m 28s
cpp-beam_search_causal_lm-Qwen1_5-7B-Chat	4m 27s
cpp-beam_search_causal_lm-Qwen-7B-Chat	4m 44s
cpp-beam_search_causal_lm-ubuntu	6m 38s
cpp-greedy_causal_lm-ubuntu	4m 13s
cpp-Phi-1_5	4m 4s
cpp-speculative_decoding_lm-ubuntu	10m 29s
	1h 1m 19s

Provide feedback