Initialize BaseOnlineStrategy

[HamedHemati]

This PR initializes BaseOnlineStrategy. As discussed before, this is meant to be considered as a possible solution(and not necessarily the final soliton) for the implementation of online strategies. Three new files are added:

avalanche/training/strategies/base_online_strategy -> the base strategy for online strategies
avalanche/training/strategies/strategy_wrappers_online.py -> tried to avoid mixing strategy wrappers
examples/online_naive -> an example for testing

[coveralls]

Pull Request Test Coverage Report for Build 1726076854

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.01%) to 79.914%

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Totals
Change from base Build 1707322059:	0.01%
Covered Lines:	11570
Relevant Lines:	14478

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💛 - Coveralls

[AntonioCarta]

Thanks @HamedHemati!

As we discussed at the previous meeting, there are many issues with this approach that need to be addressed. I think we can accept this PR to use it as a starting point. I will open a separate issue to provide a roadmap for the next steps.