Convert numpy arrays to lists before saving the evaluation metrics as json
eval_metrics contains:
    - mean_iou: float
    - mean_accuracy: float
    - overall_accuracy: float
    - per_category_iou: ndarray of shape (num_labels,)
    - per_category_accuracy: ndarray of shape (num_labels,)
ndarrays are to be converted to lists for json serialization.

@sgugger Can you please review this small update.<|||||>_The documentation is not available anymore as the PR was closed or merged._<|||||>> LGTM! Can you just run `make style` on your branch to fix the styling issue?

yes, the styling issue is now fixed.