Add Prod transform, Add test_factory

lerobot/scripts/train.py

@@ -50,6 +50,8 @@ def train(cfg: dict):
     offline_buffer = make_offline_buffer(cfg)
 
     if cfg.balanced_sampling:
+        num_traj_per_batch = cfg.batch_size
+
         online_sampler = PrioritizedSliceSampler(
             max_capacity=100_000,
             alpha=cfg.per_alpha,