Refactor(whisper): update model instantiation and loading process #1166

Job	Run time
style	47s
core	1m 14s
tests (STT, mlx_audio/stt/tests)	1m 36s
tests (STS, mlx_audio/sts/tests)	4m 39s
modular (tts, from mlx_audio.tts.utils import load_model)	50s
tests (TTS, mlx_audio/tts/tests)	1m 49s
tests (VAD, mlx_audio/vad/tests)	1m 11s
modular (stt, from mlx_audio.stt.utils import load_model)	1m 6s
modular (sts, from mlx_audio.sts.voice_pipeline import VoicePipeline)	1m 12s
tests (Codec, mlx_audio/codec/tests)	1m 24s
	15m 48s

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