Changes for ONNX conversion of Conformer models
* Brought in the needed changes from Vahid's earlier PR: https://github.com/NVIDIA/NeMo/pull/2359/files
* Tested conversion with scripts/export/convasr_to_single_onnx.py
* Tested the converted large, medium and small models with dev-clean and WER is very close (within 0.01%) of correspondiong .nemo models
