Improved Mean Error for DDP

[KickItLikeShika]

Fixes part of #1284

Made mean error metric compatible with DDP and add the necessary tests. And added an integration tests.

Check list:

• New tests are added (if a new feature is added)
• New doc strings: description and/or example code are in RST format
• Documentation is updated (if required)

[vfdev-5]

@KickItLikeShika thanks for the PR !

[vfdev-5]

@KickItLikeShika seems like GPU tests wont pass: https://app.circleci.com/pipelines/github/pytorch/ignite/1993/workflows/4c558b7a-dc4d-48fd-bc89-e8fda3404d23/jobs/6059
(this is the PR that will fix the issue with FID/IS that currently is failing and stopping other metric tests)

[KickItLikeShika]

Thanks for reporting that, unfortunately after trying to avoid sending follow-up PRs, we need to send another one, and add tolerence for distributed integration test, I will send the PR in minutes