.nemo to single .onnx CLI converter script for ASR models
Signed-off-by: Sergei Nikolaev <snikolaev@nvidia.com>

Now we can do
```
$ python convasr_to_single_onnx.py --nemo_file SpeakerNet.nemo --onnx_file SpeakerNet.onnx --model_type speaker
```