whisper : initialize decoder's rng with unique seed (#2932)

This change initializes each decoder's random number generator with a
unique seed.

The motivation for this is that currently all decoders are initialized
with the same seed value, 0. The result of this is that for the same
state (logits, probs, and logprobs) they will produce the same output.

src/whisper.cpp

@@ -5528,7 +5528,7 @@ int whisper_full_with_state(
         decoder.logprobs.resize(ctx->vocab.n_vocab);
         decoder.logits_id.reserve(ctx->model.hparams.n_vocab);
 
-        decoder.rng = std::mt19937(0);
+        decoder.rng = std::mt19937(j);
     }
 
     // the accumulated text context so far