whisper : use ggml_backend_sched (whisper/2239)
* whisper : use ggml_backend_sched (wip)
* use sched in whisper_allocr
* whisper : single backend in whisper_context
* whisper : remove whisper_state->backends_used
* whisper : remove whisper_context->backend
* whisper : reset scheduler after init
* whisper : fix external encoder (e.g. CoreML)
* whisper : cleanup
* whisper : handle null GPU buffer types + fix sycl
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Co-authored-by: slaren <slarengh@gmail.com>
