Error when exporting mixer_tts model to onnx model
**Describe the bug**

I got this error when exporting mixer_tts model to onnx model:
AttributeError: 'MixerTTSModel' object has no attribute 'input_example'

**Steps/Code to reproduce bug**

```
from nemo.collections.tts.models import MixerTTSModel
spec_generator = MixerTTSModel.from_pretrained("tts_en_lj_mixerttsx")

spec_generator.export('mixter_tts.onnx')
```

**Expected behavior**

onnx model generated succefully

**Environment overview (please complete the following information)**

pip3 install nemo_toolkit['all']

NeMo version is 1.6.0
