Dense with optional reshape

[mcabbott]

Fixes #282, by allowing Dense((5, 3), (2, 7)) as suggested. Also closes #293, #617.

For d = Dense(15, 14) I wanted to leave the original behaviour alone, but attempting to do this by dispatch landed me in ambiguity hell, so it's an if statement (which I think ought to get compiled out). Possibly this should have && ndims(x)<=2 so that such d does allow higher-dimensional data.

I also added a line to allow Dense(15 => 14) and Dense((5, 3) => (2, 7)), more like Conv and less confusing I think. But will remove if anyone objects / thinks this deserves its own issue.

Note by the way the following ::Any problem. Since this persists after ] free Flux, I don't think it's my fault. (This is on Julia 1.1)

julia> d = Dense(5,2)
Dense(5 => 2)

julia> @code_warntype d(rand(Float32, 5))
Body::Any

[DhairyaLGandhi]

This is interesting! We should also add a depwarn to the changed API.

[mcabbott]

Glad someone likes =>! But what I propose just to leave Dense(5,2) working, so that nothing breaks. If that's not too confusing?

I can add a news line, but should it go under v0.10.0?

[CarloLucibello]

Shouldn't we have in this docstring, or as separete docstrings, also the other constructors?

Dense(in, out)
Dense(W, b)

[mcabbott]

See if you like this now. I’ve also tried to make clearer that σ is not a keyword.

[mancellin]

What is the status of this?
I've been drafting a small PR with some improvements of Dense docstring, but most of it would actually be fixed (or outdated) by this PR from @mcabbott.

[mcabbott]

Would still be nice, IMO, but didn't gain traction?

I guess an alternative would be to make a nice Reshape() layer, this could even support the same notation if you defined e.g.
Dense((2,3) => (4,5)) = Chain(Reshape((2,3) => 6), Dense(6,20), Reshape(20 => (4,5))).

Instead of bias=false, there's also a proposal in #873 to write bias=Zeros().

[CarloLucibello]

+1 for the Reshape layer, to me it seems more manageable and of broader utility

[bhvieira]

+1 for the Reshape layer as well