address review suggestion: add logging for update falling back to fit

Author: ablaom

src/MLJDecisionTreeInterface.jl

@@ -150,6 +150,8 @@ function MMI.update(
     only_iterations_have_changed = MMI.is_same_except(model, old_model, :n_trees)
 
     if !only_iterations_have_changed
+        verbosity > 0 && @info "Detected a change to hyperparameters " *
+            "not restricted to `n_trees`. Recomputing $n_trees trees. "
         return MMI.fit(
             model,
             verbosity,

test/runtests.jl

@@ -281,12 +281,16 @@ end
             fit!(mach, verbosity=0)
             forest1_4 = fitted_params(mach).forest
             @test length(forest1_4) ==4
+
+            # increase n_trees:
             mach.model = $M(n_trees=7, rng = stable_rng())
             @test_logs(
                 (:info, r""),
                 (:info, r"Adding 3 trees"),
                 fit!(mach, verbosity=1),
             )
+
+            # decrease n_trees:
             mach.model = $M(n_trees=5, rng = stable_rng())
             @test_logs(
                 (:info, r""),
@@ -295,6 +299,16 @@ end
             )
             forest1_5 = fitted_params(mach).forest
             @test length(forest1_5) == 5
+
+            # change a different hyperparameter:
+            mach.model = $M(n_trees=5, rng = stable_rng(), max_depth=1)
+            @test_logs(
+                (:info, r""),
+                (:info, r"Detected"),
+                fit!(mach, verbosity=1),
+            )
+            forest1_5_again = fitted_params(mach).forest
+            @test length(forest1_5_again) == 5
         end |> eval
     end
 end