[Enhancement] Upgrade isort in pre-commit hook (#1270)

.pre-commit-config.yaml

mmseg/datasets/builder.py

setup.cfg

tests/test_config.py

tests/test_metrics.py

tests/test_models/test_losses/test_ce_loss.py

tests/test_models/test_losses/test_dice_loss.py

tests/test_models/test_losses/test_lovasz_loss.py

tools/pytorch2onnx.py