give an option to store EpochOutputStore data on engine.state

[radekosmulski]

Implements #1939

Description: When EpochOutputStore is attached and a name is provided, at EPOCH_COMPLETED engine.state.{name} is set to eos.data (in other words, eos.data is stored on engine.state).

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)

[sdesrozis]

@radekosmulski Thank you ! Please check my suggestion.

[vfdev-5]

@radekosmulski please also update the docstring here.
Let's also put .. versionchanged:: as it is explained here : https://github.com/pytorch/ignite/blob/master/CONTRIBUTING.md#writing-documentation
We can put 0.5.0 version.

[vfdev-5]

@radekosmulski thanks for the PR, looks good, just a docstring update remains. If you would like to move this class into core in another PR, it is OK for me (maybe even prefered way to do that).

[radekosmulski]

I added the changes in two commits - one updates the docstrings, the other the example. I am not sure if the update to the example is needed?

Generally, not sure if this is the right way to do it 🙂

Will move it out of contrib in a separate PR 🙂

[vfdev-5]

LGTM! Thanks @radekosmulski