Expose ItemLazy to be able to implement for custom types (tracel-ai#2525

)

* Expose ItemLazy to be able to implement for custom types

* Add comment

crates/burn-train/src/metric/mod.rs

@@ -41,6 +41,9 @@ pub use memory_use::*;
 pub use top_k_acc::*;
 
 pub(crate) mod processor;
+// Expose `ItemLazy` so it can be implemented for custom types
+pub use processor::ItemLazy;
+
 /// Module responsible to save and exposes data collected during training.
 pub mod store;