Is there a list of models that can export ONNX?
'vad_multilingual_frame_marblenet' can.t export. is there a restriction on exporting models for some reason?

```python
import nemo.collections.asr as nemo_asr
model = nemo_asr.models.EncDecFrameClassificationModel.from_pretrained(model_name="vad_multilingual_frame_marblenet")

model.export('test.onnx', onnx_opset_version=12)
```
error:
'EncDecFrameClassificationModel' object has no attribute 'output_names'