Add gradient mean, std and metasplit

eval/plot.py

src/decentralizepy/datasets/Celeba.py

src/decentralizepy/node/Node.py

src/decentralizepy/sharing/PartialModel.py

src/decentralizepy/sharing/Sharing.py

src/decentralizepy/training/GradientAccumulator.py

src/decentralizepy/training/Training.py