Improve Metric Module

PyTorch-Ignite is a great library built on top of pytorch. It provides an extremely simple engine and event system to work with, which work really well in a distributed environment. It also has a set of very handy metrics which can be used right out of the box. This is why I would like to become a contributor for the PyTorch-Ignite library. I wish to work on the metrics module because I have worked in Brain-Computer Interface, Computer Vision, Augmented Reality and am familiar with a myriad of different metrics. I am also familiar with using pytorch library and have built, trained and tested many models with it, making me a suitable candidate to work on the metrics module of PyTorch-Ignite. Contributing to PyTorch-Ignite can help me learn more about using pytorch in a distributed environment, writing test cases for your code, get used to the GitHub workflow in general.