Rename Epoch to Iterations when using epoch_length with max_epochs=1

## 🚀 Feature

Per the discussion in https://github.com/pytorch/ignite/issues/1263, we'd like to ensure that the progress bar uses the name "Iterations" instead of "Epoch" when the `epoch_length` is passed to the trainer for `max_epochs=1`.