Recurrent difficulties

[DrChainsaw]

Flux uses sequences for recurrent layers while ONNX wants 3D input.

Current workaround is to

1. Specify inputs to recurrent as 3D
2. Reshape/ flatten time dimension as batch dimension if hitting a Dense layer with 3D input

1 is not so good as the general API is to specify input shapes in the Flux format.
2 is ok-ish, but it is not really an honest representation of what is going on in flux and it does create a fair bit of look-ahead-a-few-layers-and-figure-out-if-a-few-OPs-shall-be-ignored wonderfulness.

Also, Dense -> Recurrent does not work iirc.

Perhaps something can be done with sequences, although I dread trying to figure out in deserialization if something is wrapped in a for loop over elements of a sequence or whatever one needs to do to feed a sequence into GEMM.

Perhaps the easiest way out is to just give in and use a 3D->sequence wrapper around recurrent layers but I can't imagine this being good for performance.