data_loader: transform/targetTransform fields are declared but never invoked

`dataLoader_t.transform` / `targetTransform` are stored by `initDataLoader` and documented in DataLoader.h ("used once after loading the dataset" / "after each getBatch"), but no code path ever calls them (found during #326 exploration; re-confirmed at implementation time). Either wire them up or remove the dead seams — today they read as extension points but silently do nothing.

Start with DataLoader.h and the initDataLoader entry point, then trace where transform and targetTransform are stored and where dataset loading and getBatch are implemented. Confirm whether the documented invocation points can be reached; done means the callbacks work as documented, with coverage for both timing points, or the unused fields and documentation are removed.

Written by the indexing model from the issue text.