ONNX: Gather for tensors of rank > 0.

Firstly thanks for candle, it's an impressive project and the speed at which updates are coming is really cool as a user!

I'm trying to use a speedyspeech onnx model in it right now and there's a failing op:

> not yet implemented: implement gather for Tensor[dims 78, 128; f32] Tensor[dims 1, 16; i64] axis 0

I can provide my ONNX or assist via PRs if required (may need some pointers). Otherwise just wondering if there's any idea when Gather might fully land or if there are any difficulties in the implementation that might mean I should look for an alternative solution in the near term.

Start by reproducing the speedyspeech ONNX model failure and tracing the unimplemented Gather operation for rank-greater-than-zero tensors. Done means the reported Gather case no longer returns a not-implemented error and the model can proceed past this operation.

Written by the indexing model from the issue text.