Removing deprecated method call in GPU_BACKEND_ORDER.

[codetalker7]

Closes #2312.

[ToucheSir]

Nice, this is cleaner than my ideas!

[prbzrg]

This isn't same as before, it should be OrderedDict from OrderedCollections.

julia> const GPU_BACKENDS = ["CUDA", "AMD", "Metal", "CPU"]
4-element Vector{String}:
 "CUDA"
 "AMD"
 "Metal"
 "CPU"

julia> const GPU_BACKEND_ORDER = Dict(collect(zip(GPU_BACKENDS, 1:length(GPU_BACKENDS))))
Dict{String, Int64} with 4 entries:
  "CUDA"  => 1
  "AMD"   => 2
  "CPU"   => 4
  "Metal" => 3

julia> const GPU_BACKEND_ORDER2 = OrderedDict(collect(zip(GPU_BACKENDS, 1:length(GPU_BACKENDS))))
OrderedDict{String, Int64} with 4 entries:
  "CUDA"  => 1
  "AMD"   => 2
  "Metal" => 3
  "CPU"   => 4

[prbzrg]

on [email protected]:

julia> Flux.GPU_BACKEND_ORDER
OrderedDict{String, Int64} with 4 entries:
  "CUDA"  => 1
  "AMD"   => 2
  "Metal" => 3
  "CPU"   => 4

[ToucheSir]

GPU_BACKEND_ORDER isn't a part of the public API, nor is its iteration order defined. We're not going to add a package dependency just for this.

[prbzrg]

Oh, I was under the impression that the order of it matters because there was a sort that caused the deprecation warning.