1. removed `export2.py` from aishell recipe
2. removed `git lfs` commands for pulling `tokens.txt` from `test-onnx-export.sh`
3. applied #1152 to `export-onnx-streaming.py` and `export-onnx.py`
details are listed below:
1. updated `git lfs` command in `export-onnx.py` and `onnx_pretrained-streaming.py`;
2. added rounding code to `export.py` for case where `params.avg == 1`.