Read default backend from application environment

nx/lib/nx.ex

@@ -277,17 +277,24 @@ defmodule Nx do
         Nx.exp(t) / Nx.sum(Nx.exp(t))
       end
 
-  The `keep_on_device: true` run option will keep the tensor on
-  the backend. You can transfer it back to a binary tensor by
-  calling `backend_transfer/2` or `backend_copy/2`. If you don't
-  intend to use the data for some reason, you can explicitly call
-  `backend_deallocate/1` to deallocate it.
+  The `keep_on_device: true` run option will keep the tensor
+  allocated elsewhere and not as an Elixir binary. You can transfer
+  it back to a binary tensor by calling `backend_transfer/2` or
+  `backend_copy/2`. If you don't intend to use the data for some
+  reason, you can explicitly call `backend_deallocate/1` to
+  deallocate it.
 
   However, most often backends are used to provide a completely
   different implementation of tensor operations, often accelerated
   to the GPU. In such cases, you want to guarantee all tensors
-  are allocated in the new backend. This can be done by calling
-  `Nx.default_backend/1`:
+  are allocated in the new backend. This can be done by configuring
+  your runtime:
+
+      # config/runtime.exs
+      import Config
+      config :nx, default_backend: Lib.CustomBackend
+
+  Or by calling `Nx.default_backend/1`:
 
       Nx.default_backend({Lib.CustomBackend, device: :cuda})
 
@@ -2444,7 +2451,6 @@ defmodule Nx do
   ## Backend API
 
   @backend_key {Nx, :default_backend}
-  @backend_default {Nx.BinaryBackend, []}
 
   @doc """
   Sets the current process default backend to `backend` with the given `opts`.
@@ -2462,14 +2468,15 @@ defmodule Nx do
 
   """
   def default_backend(backend) do
-    Process.put(@backend_key, backend!(backend)) || @backend_default
+    Process.put(@backend_key, backend!(backend)) ||
+      backend!(Application.fetch_env!(:nx, :default_backend))
   end
 
   @doc """
   Gets the default backend for the current process.
   """
   def default_backend() do
-    Process.get(@backend_key) || @backend_default
+    Process.get(@backend_key) || backend!(Application.fetch_env!(:nx, :default_backend))
   end
 
   @doc """

nx/mix.exs

@@ -20,7 +20,10 @@ defmodule Nx.MixProject do
   end
 
   def application do
-    [extra_applications: [:logger]]
+    [
+      extra_applications: [:logger],
+      env: [default_backend: {Nx.BinaryBackend, []}]
+    ]
   end
 
   defp deps do