Add support for networks with multiple outputs in ONNX exporter

[ptrendx]

Description

Add support for networks with multiple outputs to ONNX exporter

Checklist

Essentials

Please feel free to remove inapplicable items for your PR.

• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage:
• Unit tests are added for small changes to verify correctness (e.g. adding a new operator)
• To the my best knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change

[nswamy]

@Roshrini

[lupesko]

Thanks for the contribution @ptrendx !
@anirudhacharya @vandanavk - to take a look as well.

[anirudhacharya]

Please add a test in this file - https://github.com/apache/incubator-mxnet/blob/master/tests/python-pytest/onnx/export/mxnet_export_test.py using a model with multiple outputs.

[stu1130]

@ptrendx bounce again! Could you please add a test! Thanks

[vandanavk]

@mxnet-label-bot [pr-awaiting-response]

[BobLiu20]

Waiting for this PR! Thanks.

[Roshrini]

@ptrendx ping again :)

[ankkhedia]

@ptrendx Thanks for the contribution! :)
Could you please look into the review comments?

[kalyc]

@mxnet-label-bot update [pr-awaiting-review]

[stu1130]

@ptrendx thanks for addressing the comment. @anirudhacharya @Roshrini @vandanavk could you please take a look at this? Thanks

[vandanavk]

@ptrendx Sina submitted a PR #13390 for multi outputs. Please have a look

[ptrendx]

Closing then since it achieves the same thing.