whisper : fixed Beam Search Strategy and exposed whisper_pcm_to_mel_phase_vocoder (#474)

Co-authored-by: Sandro Hanea <sandrohanea@microsoft.com>

whisper.h

@@ -113,6 +113,16 @@ extern "C" {
                                int   n_samples,
                                int   n_threads);
 
+    // Convert RAW PCM audio to log mel spectrogram but applies a Phase Vocoder to speed up the audio x2. 
+    // The resulting spectrogram is stored inside the provided whisper context.
+    // Returns 0 on success
+    WHISPER_API int whisper_pcm_to_mel_phase_vocoder(
+        struct whisper_context* ctx,
+        const float* samples,
+        int   n_samples,
+        int   n_threads);
+
+
     // This can be used to set a custom log mel spectrogram inside the provided whisper context.
     // Use this instead of whisper_pcm_to_mel() if you want to provide your own log mel spectrogram.
     // n_mel must be 80