refactor: reorganize sampler module and update imports

src/sampler/tpe/mod.rs

@@ -4,9 +4,13 @@
 //! including support for intersection search space calculation.
 
 mod gamma;
+mod multivariate;
 mod sampler;
 pub mod search_space;
 
 pub use gamma::{FixedGamma, GammaStrategy, HyperoptGamma, LinearGamma, SqrtGamma};
+pub use multivariate::{
+    ConstantLiarStrategy, MultivariateTpeSampler, MultivariateTpeSamplerBuilder,
+};
 pub use sampler::{TpeSampler, TpeSamplerBuilder};
 pub use search_space::{GroupDecomposedSearchSpace, IntersectionSearchSpace};

src/sampler/tpe/sampler.rs

@@ -280,6 +280,7 @@ impl TpeSampler {
         clippy::cast_possible_truncation,
         clippy::cast_sign_loss
     )]
+    #[must_use]
     fn split_trials<'a>(
         &self,
         history: &'a [CompletedTrial],