whisper : add whisper_state + default state on the whisper_context (#523)

whisper : add whisper_state + default state on the whisper_context (#523)

* Added whisper state + default state on the whisper_context

* Fixed some examples and bindings

* Fixed whisper_n_len (which was used in some binding) and added whisper_n_len_from_state

* Fixed comments

* whisper : reuse kv_cache_free() and fix compiler warnings

* whisper : clean-up the API comments

---------

Co-authored-by: Sandro Hanea <redacted>
Co-authored-by: Georgi Gerganov <redacted>