Single-argument Flux.outdims for layers with fixed output dimension?

[lassepe]

As of #1252 calls to Flux.outdims(l::Dense, isize) check that isize machtes the input dimension of the dense layer. I appreciate this check and think it is the right thing to do if isize is provided.

However, as the Dense layer has fixed input and output dimensions it might be convenient to also provide a single-argument outdims method (there are probably also other layers that would benefit from this). Of course, users could also directly check for the dimensions of the weights l.W to figure this out but field access feels like a rather unsafe way to achieve this since the field names are implementation details that may change.

While we are at it, it may also be useful to add Flux.indims for those fixed-dimension layers.

If this sounds like a useful addition, I am happy to create a PR.

[AliMalik9599]

Hi @CarloLucibello, I am interested in working on enhancing this feature. I'm relatively new to Julia, any advice on getting started? Thanks!