whisper : add batched decoding (#1486)

whisper : add batched decoding (#1486)

* whisper : add whisper_batch

* whisper : move kv_self to whisper_state

* whisper : full batched decoding support

* whisper : fix memory leak in whisper_batch

* whisper : fix mem leak again + remove oboslete function

* whisper : clear kv cache when using whisper_decode API

* whisper : speed-up sampling

* whisper : fix decoders initializer

* bench : add batch size 5 bench

* whisper : add comment about the KV cache size

* whisper : add check for max number of decoders

* whisper : avoid starting sampling threads with bs=1

* whisper : enable beam-search by default

* cuda : sync llama.cpp fixes