doctest for epoch metric (#2351)

Co-authored-by: Desroziers <sylvain.desroziers@michelin.com>

ignite/metrics/epoch_metric.py

@@ -40,6 +40,24 @@ class EpochMetric(Metric):
             Default, True.
         device: optional device specification for internal storage.
 
+    Example:
+
+        .. testcode::
+
+            def mse_fn(y_preds, y_targets):
+                return torch.mean(((y_preds - y_targets.type_as(y_preds)) ** 2)).item()
+
+            metric = EpochMetric(mse_fn)
+            metric.attach(default_evaluator, "mse")
+            y_true = torch.Tensor([0, 1, 2, 3, 4, 5])
+            y_pred = y_true * 0.75
+            state = default_evaluator.run([[y_pred, y_true]])
+            print(state.metrics["mse"])
+
+        .. testoutput::
+
+            0.5729...
+
     Warnings:
         EpochMetricWarning: User is warned that there are issues with ``compute_fn`` on a batch of data processed.
         To disable the warning, set ``check_compute_fn=False``.