whisper : token-level timestamps with DTW (whisper/1485)

whisper : token-level timestamps with DTW (whisper/1485)

* whisper.cpp: impl dtw algo

* WIP: producing and placing DTW timestamps on tokens

* Fix compile and assertion errors. Attempt to DTW timestamp with single_segment=false.

* Fix mistake causing incorrect alignment of dtw timestamps

* implement N_TOP_MOST and CUSTOM alignment heads setting

* whisper: fix typo on alignment heads enum

* Fix issues related to changes in whisper.cpp

* Fixed excessive memory use when using DTW timestamps. Other minor fixes to DTW timestamping function

* decoder: save cross QKs only if requested

* Calling median filter with ggml_map_custom1

* Reimpl aheads n_top_most and custom. Sanity checks on chosen aheads

* Copying cross QKs from decoder backend correctly

* dtw: cleanup

* Fix incorrect n_frames passed to dtw when near end of audio

* Fix aheads_masks_init for backend != CPU

* whisper : minor style

* main : add dtw (wip)

* whisper: fix invalid memory access in aheads_masks_init

* main : add dtw (cont)

* whisper : minor

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Co-authored-by: Georgi Gerganov <redacted>