[v1.x] ONNX 1.6 compatibility fix + fix for when multiple nodes have the same name

[Zha0q1]

This pr fixes:

1. softmax when axis == -1
2. compatibility with onnx 1.6
3. adds a prefix (index) to the exported nodes so that we can handle when multiple nodes have the same name

[mxnet-bot]

Hey @Zha0q1 , Thanks for submitting the PR
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[josephevans]

LGTM, thanks!