ECAPA TDNN pretrained fails to export ONNX - missing 'input_example'
**Describe the bug**

I cannot export the pertained ECAPA TDNN model for embeddings to .onnx 
Exporting  requires the model to have a `input_example` which is not available on the ecapa tdnn model.

**Steps/Code to reproduce bug**

```
import nemo.collections.asr as nemo_asr
speaker_model = nemo_asr.models.EncDecSpeakerLabelModel.from_pretrained(model_name='ecapa_tdnn')
speaker_model.export(output='mymodel.onnx')
```

Output:
```
TypeError                                 Traceback (most recent call last)
<ipython-input-51-d55b2004ba1e> in <module>()
      3 import nemo.collections.asr as nemo_asr
      4 speaker_model = nemo_asr.models.EncDecSpeakerLabelModel.from_pretrained(model_name='ecapa_tdnn')
----> 5 speaker_model.export(output='mymodel.onnx')

1 frames
/usr/local/lib/python3.7/dist-packages/nemo/core/classes/exportable.py in _setup_input_example(self, input_example)
    389 
    390     def _setup_input_example(self, input_example):
--> 391         input_list = list(input_example)
    392         input_dict = {}
    393         # process possible kwargs

TypeError: 'NoneType' object is not iterable

```

**Expected behavior**

I get a mymodel.onnx

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

 - Environment location: [Bare-metal, Docker, Cloud(specify cloud provider - AWS, Azure, GCP, Collab)]
 - Method of NeMo install: [pip install or from source]. Please specify exact commands you used to install.
 - If method of install is [Docker], provide `docker pull` & `docker run` commands used

**Environment details**

Nemo version: r1.5.0

**Additional context**

GPU: Tesla T4
