lookahead : add example for lookahead decoding (#4207)
* lookahead : init
* lookahead : generate and store n-grams
* lookahead : use loop instead recursion to generate n-grams
* lookahead : initial working implementation
* lookahead : filter repeating n-grams
* lookahead : use deterministic init
* lookahead : add to Makefile
* lookahead : fix a bug in the seq_id of the lookahead tokens
* lookahead : add comments
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Co-authored-by: slaren <slarengh@gmail.com>
