Updates to conformer.
@borisfom or @drnikolaev would you please checkout the asr_model.py? 
I needed to make some changes to the export part. We did not pass the length to the encoder and it was fine for convolutional modules which do not use that input. But Conformer would use that input and export would not work on this model without passing lengths.
+ Made positional embedding more efficient.
+ Updated the rel_shift() method to avoid flipping which needs significant memory.
+ Fixed the onnx issue.
+ Added att_context_size feature to limit the context of attention with masking.