added topk patch

[scumatteo]

If top-k metric is called with num_classes < k, top-k metric returns 0.

[coveralls]

Pull Request Test Coverage Report for Build 5077487805

• 1 of 2 (50.0%) changed or added relevant lines in 1 file are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.1%) to 72.217%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
avalanche/evaluation/metrics/topk_acc.py	1	2	50.0%

Files with Coverage Reduction	New Missed Lines	%
avalanche/evaluation/metrics/amca.py	2	75.0%

Totals
Change from base Build 5065785914:	0.1%
Covered Lines:	15749
Relevant Lines:	21808

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💛 - Coveralls

[AntonioCarta]

It doesn't make a lot of sense to return 0 to mask an error, especially at the metric-level. IMO the plugin (not the metric) should not emit values when num_classes < k. If called, the metric should still fail.