integrate statistics from pie template repo

[ArneBinder]

from here: https://github.com/ChristophAlt/pytorch-ie-hydra-template/blob/main/src/utils/statistics.py
create module document.statistics

EDIT: This should make use of #43 pytorch_ie.core.DocumentMetric, see ArneBinder/pytorch-ie#308.

[ArneBinder]

This was already integrate into pytorch-ie, see ArneBinder/pytorch-ie#312.