EarlyStopping based no improvement interval

[irumata]

Types of changes

TL;DR

Early stop when no loss reduces during tolerance_window asks Example of using : optimizer.register_callback("ask", ng.callbacks.EarlyStopping.no_improvement_stopper(no_imp_window))

tested python3 -m pytest test_callbacks.py

• [*] New feature (non-breaking change which adds functionality)

Motivation and Context / Related issue

Provide early stoping feedback based on no improvemen frequent case for optimization

there are several issues connected to that early stoping implementation https://github.com/facebookresearch/nevergrad/issues/714 https://github.com/facebookresearch/nevergrad/issues/589

How Has This Been Tested (if it applies)

I've provided different unit tests to test it python3 -m pytest test_callbacks.py

Checklist

• [ ?] The documentation is up-to-date with the changes I made: there no place on github to add it
• [ x] I have read the CONTRIBUTING document and completed the CLA (see CLA).
• [ x] All tests passed, and additional code has been covered with new tests.