Add prompt lookup decoding #595

Job	Run time
cpp-beam_search_causal_lm-ubuntu	7m 16s
cpp-beam_search_causal_lm-notus-7b-v1	4m 42s
cpp-beam_search_causal_lm-Qwen-7B-Chat	7m 17s
cpp-beam_search_causal_lm-Qwen1_5-7B-Chat	5m 13s
cpp-beam_search_causal_lm-Phi-2	4m 56s
cpp-beam_search_causal_lm-windows	14m 21s
cpp-greedy_causal_lm-ubuntu	4m 49s
cpp-Phi-1_5	2m 58s
cpp-prompt_lookup_decoding_lm-ubuntu	4m 14s
cpp-speculative_decoding_lm-ubuntu	9m 57s
cpp-greedy_causal_lm-redpajama-3b-chat	7m 54s
	1h 13m 37s

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