Revert "Using serde (serde_pyo3) to get __str__ and __repr__ easily."

This reverts commit 86138337fc.

bindings/python/src/trainers.rs

@@ -16,8 +16,8 @@ use tokenizers as tk;
 /// Trainer will return an instance of this class when instantiated.
 #[pyclass(module = "tokenizers.trainers", name = "Trainer", subclass)]
 #[derive(Clone, Deserialize, Serialize)]
-#[serde(transparent)]
 pub struct PyTrainer {
+    #[serde(flatten)]
     pub trainer: Arc<RwLock<TrainerWrapper>>,
 }