Do not use EXLA at compile-time (#167)

.github/workflows/test.yml

@@ -47,3 +47,5 @@ jobs:
         if: ${{ matrix.lint }}
 
       - run: mix test
+
+      - run: USE_EXLA_AT_COMPILE_TIME=1 mix compile --force

config/config.exs

@@ -1,3 +1,7 @@
 import Config
 
 config :exla, :add_backend_on_inspect, config_env() != :test
+
+if System.get_env("USE_EXLA_AT_COMPILE_TIME") do
+  config :nx, :default_backend, EXLA.Backend
+end

lib/scholar/metrics/classification.ex

@@ -67,7 +67,7 @@ defmodule Scholar.Metrics.Classification do
   brier_score_loss_schema = [
     sample_weights: [
       type: {:custom, Scholar.Options, :weights, []},
-      default: Nx.tensor(1.0),
+      default: 1.0,
       doc: """
       Sample weights of the observations.
       """

mix.exs

@@ -32,7 +32,7 @@ defmodule Scholar.MixProject do
       {:ex_doc, "~> 0.30", only: :docs},
       {:nx, "~> 0.6"},
       {:nimble_options, "~> 0.5.2 or ~> 1.0"},
-      {:exla, "~> 0.6"},
+      {:exla, "~> 0.6", optional: true},
       {:polaris, "~> 0.1"}
     ]
   end