🚀 Feature

I'd like to implement the mean absolute percentage error (MAPE) in ignite/metrics.

It is a commonly used metric for regression problems and it would be really convenient to be able to use it directly with ignite evaluators.

For that, I would write a custom Metric class in a new file mean_absolute_percentage_error.py inheriting from the base Metric class in ignite/metrics/metric.py.