Add `return_timestamps` attribute to generation config

For consistency with the other Whisper models, we should set the `return_timestamps` attribute to `False` in the generation config, e.g.: for Whisper Small: https://huggingface.co/openai/whisper-small/blob/e34e8ae444c29815eca53e11383ea13b2e362eb0/generation_config.json#L167

cc

@patrickvonplaten
if you could merge that would be grand! Thanks!

generation_config.json

@@ -148,6 +148,7 @@
   "max_length": 448,
   "no_timestamps_token_id": 50363,
   "pad_token_id": 50257,
+  "return_timestamps": false,
   "suppress_tokens": [
     1,
     2,