Implement operator `Gather` for backend `Gorgonia`

[mihard]

Why is this operator needed?

Without this basic operator, it's impossible to run many NLP models with onnx-go.

Implementation

• Description of the Gather operator in ONNX

Link to existing material on the backend

N/A

Expected problems?

N/A

Tests

go test -run=ONNX/TestOperator

[Prayforhanluo]

same issue

[inesani]

same issue here

[kwhtsng]

same issue

[godshen]

same issue

[eccentrico]

same issue here