Add an explanation on how metrics work with engine #2437
Description
📚 Documentation
It is unclear for newcomers what does the code below from docs (for example, Accuracy)
metric = Accuracy()
metric.attach(default_evaluator, "accuracy")
y_true = torch.Tensor([1, 0, 1, 1, 0, 1])
y_pred = torch.Tensor([1, 0, 1, 0, 1, 1])
state = default_evaluator.run([[y_pred, y_true]])
print(state.metrics["accuracy"])More precisely, it is unclear that the metric is computed online and at each iteration some internal variables are updated and on epoch completed metric value is computed.
For each metric docs, let's add a link on the explanation that we can add explicitly to https://pytorch.org/ignite/metrics.html