add function fleaves

[CarloLucibello]

Adds utility function fflatten fleaves that flatten the leaves into an array, similarly to jax's tree_leaves.
Useful for models' parameters collection:

julia> using Flux

julia> m = Chain(Dense(2=>3, relu), Dense(3 => 2))
Chain(
  Dense(2 => 3, relu),                  # 9 parameters
  Dense(3 => 2),                        # 8 parameters
)                   # Total: 4 arrays, 17 parameters, 324 bytes.

julia> fleaves(m)
6-element Vector{Any}:
 Float32[-0.0102046095 -0.28821245; 0.4982538 0.23464581; -0.70785844 0.3291903]
 Float32[0.0, 0.0, 0.0]
 relu (generic function with 2 methods)
 Float32[0.57762665 -0.8608131 -0.5159058; -0.54350173 0.61605966 0.8265723]
 Float32[0.0, 0.0]
 identity (generic function with 1 method)

[CarloLucibello]

no functional changes here, only improved clarity

[darsnack]

Implementation looks good and I'm on board with adding something like this. Only thing is that this isn't jtu.tree_flatten. If it were, it should return the analog to a tree def as well. I would say jtu.tree_flatten is more like Optimisers.restructure except without concatenating the leaves.

It'd be nice to have something like a real jtu.tree_flatten, but for now I suggest renaming this to fleaves to match jtu.tree_leaves—which is what this actually is.

[CarloLucibello]

Changed name to fleaves

[CarloLucibello]

this is ready to go