Set decoder_start_token_id and output_past in config

Without the `decoder_start_token_id` parameter, you get the following `ValueError` while using the model:

```
561 elif (
562 hasattr(self.config, "decoder")
563 and hasattr(self.config.decoder, "bos_token_id")
564 and self.config.decoder.bos_token_id is not None
565 ):
566 return self.config.decoder.bos_token_id
--> 567 raise ValueError(
568 "`decoder_start_token_id` or `bos_token_id` has to be defined for encoder-decoder generation."
569 )

ValueError: `decoder_start_token_id` or `bos_token_id` has to be defined for encoder-decoder generation.
```

I checked https://huggingface.co/google/flan-t5-large/blob/main/config.json and noticed that `output_past` is also different.

config.json

@@ -5,6 +5,7 @@
   "d_ff": 10240,
   "d_kv": 64,
   "d_model": 4096,
+  "decoder_start_token_id": 0,
   "dense_act_fn": "gelu",
   "dropout_rate": 0.1,
   "eos_token_id": 1,
@@ -17,6 +18,7 @@
   "num_decoder_layers": 24,
   "num_heads": 64,
   "num_layers": 24,
+  "output_past": true,
   "pad_token_id": 0,
   "relative_attention_max_distance": 128,
   "relative_attention_num_buckets": 32,