[BUG] Specification in ReduceMean lowering pass has a typo

[viv-eth]

Describe the bug

The new specifications of ONNX add axes as an attribute, however in the LoweringOptimizationPasses.py it is indexed as axis, which results in an error when the graph contains a ReduceMean node.

To Reproduce

Run any test with a ReduceMean operation.

Expected behavior

Generate network successfully once _remove_only_singleton_reduce_mean function is fixed.

Additional context

I will open a PR for this, leaving this as mental note.

[Xeratec]

Do you have a timeline on this, or should I include it in an upcoming PR I am preparing to fix some other stuff?