Fix Your first Axon model guide (#627)

Fix a crash when evaluating the init_fn cell. The second argument to `init_fn.`, `%Axon.ModelState{}` was a struct with `nil` fields internally, and when `Nx` tried to traverse it as a container, those `nil` atoms failed the `Nx.LazyContainer` protocol.

Author: pguyot

guides/model_creation/your_first_axon_model.livemd

@@ -82,7 +82,7 @@ predict_fn.(params :: map(tensor), input :: map(tensor) | tensor)
 `predict_fn` returns transformed inputs from your model's trainable parameters and the given inputs.
 
 ```elixir
-params = init_fn.(Nx.template({1, 8}, :f32), %Axon.ModelState{})
+params = init_fn.(Nx.template({1, 8}, :f32), Axon.ModelState.empty())
 ```
 
 <!-- livebook:{"output":true} -->