generate with tf.function (xla) not working for tf model export
### System Info

using the latest version with tf.function for generate still doesn't work for tensorflow model export which would be useful for serving.

### Who can help?

@gante @patrickvonplaten 

### Information

- [ ] The official example scripts
- [ ] My own modified scripts

### Tasks

- [ ] An officially supported task in the `examples` folder (such as GLUE/SQuAD, ...)
- [ ] My own task or dataset (give details below)

### Reproduction

```
import tensorflow as tf
from transformers import TFAutoModelForSeq2SeqLM

class MyOwnModel(tf.Module):
    def __init__(self, model_path="t5-small"):
        super(MyOwnModel, self).__init__()
        self.model = TFAutoModelForSeq2SeqLM.from_pretrained(model_path)

    @tf.function(input_signature=(tf.TensorSpec((None, 32), tf.int32, name="input_ids"),
                                  tf.TensorSpec((None, 32), tf.int32, name="attention_mask")), jit_compile=True)
    def serving(self, input_ids, attention_mask):
        return self.model.generate(input_ids=input_ids, attention_mask=attention_mask, max_new_tokens=32)

model = MyOwnModel()
export_dir = "./"
tf.saved_model.save(
    model,
    export_dir,
    signatures={
        "serving_default":
            model.serving
    })
```

error:
```
File "../python3.8/site-packages/transformers/generation_tf_utils.py", line 1561, in _generate  *
        input_ids = self._prepare_decoder_input_ids_for_generation(
    File "../python3.8/site-packages/transformers/generation_tf_utils.py", line 1758, in _prepare_decoder_input_ids_for_generation  *
        return tf.ones((batch_size, 1), dtype=tf.int32) * decoder_start_token_id

    TypeError: Expected int32, but got None of type 'NoneType'.
```

### Expected behavior

generate can be exported in tf model for serving