MyPy: improve ignite.contrib module

[gruebel]

🚀 Feature

The ignite.contrib module is the last one, which is ignored by the mypy config 💪 For anyone, who wants to do it, I would recommend to split it in 3-4 PRs

• ignite.contrib.engines
• ignite.contrib.handlers
• ignite.contrib.metrics
• (Optional separate PR) ignite.contrib.metrics.regression

After that we could also enable more stricter mypy configuration to further improve the typing of the project.

So, feel free to ask any questions here or in your PRs, I will try my best to give proper guidance and @vfdev-5 will do the same for sure, like he did with my previous PRs 👍

[zhxxn]

Hi, I would like to give it a try!

[vfdev-5]

@gruebel thanks a lot for opening the issue !
@zhxxn let me assign it to you.
As this issue is large, we can assign more than one person to the issue.

[zhxxn]

Hi! I would then start with ignite.contrib.metrics module 😄

[zhxxn]

I just finished my work on ignite.contrib.metrics.
Next I would like to try ignite.contrib.handlers 😄

[vfdev-5]

Next I would like to try ignite.contrib.handlers

@zhxxn how's going your attempt on ignite.contrib.handlers ?