Unwrap containers by default in Axon custom layers

[seanmor5]

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We should consider unwrapping containers by default in Axon custom layers. So for example if you have a custom layer:

  defn custom_layer(foo, bar, _opts) do
    {foo, bar}
  end

You can just pattern match on the output and get regular Axon structs:

  {foo_axon, bar_axon} = Axon.layer(&custom_layer/3, [foo, bar])