Fix WandB dataset tag in the case of multiple `dataset_repo_id`

[marinabar]

What this does

Update dataset tag to : 'first dataset name...' when dataset_repo_id is a list.

A WandB tag needs to be str and no more than 64 characters long. Otherwise, an error was thrown : wandb.errors.CommError: invalid tag 'dataset:['m1b/koch_pick_red_lego_new_camera', 'm1b/koch_pick_red_lego_green_box']'. must be between 1 and 64 characters (Error 400: Bad Request)

How this was tested

python lerobot/scripts/train.py \
 hydra.job.name=test_wandb_tag \
 policy=act \
 dataset_repo_id='[lerobot/aloha_sim_transfer_cube_human,lerobot/aloha_sim_insertion_human]' \
 env=aloha \
 training.offline_steps=10 \
 wandb.enable=true