fix: remove unused imports in convertor_tests.rs

These imports were causing compilation errors in non-test builds:
- test_helpers::NdArrayAutodiff (only available in test mode)
- FSRSBatcher, constant_weighted_fsrs_items (unused)
- burn::* modules (unused)
- chrono_tz::Tz, serde (unused)

Author: L-M-Sherlock

src/convertor_tests.rs

@@ -1,17 +1,7 @@
 use crate::convertor_tests::RevlogReviewKind::*;
-use crate::dataset::{FSRSBatcher, constant_weighted_fsrs_items};
 use crate::dataset::{FSRSItem, FSRSReview};
-// use crate::simulation::{RevlogEntry, RevlogReviewKind};
-use crate::test_helpers::NdArrayAutodiff;
-use burn::backend::ndarray::NdArrayDevice;
-use burn::data::dataloader::Dataset;
-use burn::data::dataloader::batcher::Batcher;
-use burn::data::dataset::InMemDataset;
-use burn::tensor::cast::ToElement;
 use chrono::prelude::*;
-use chrono_tz::Tz;
 use itertools::Itertools;
-use serde::{Deserialize, Serialize};
 
 /*
 impl rusqlite::types::FromSql for RevlogReviewKind {