Support for lazy streams in benchmark_with_validation_stream

[lrzpellegrini]

This PR introduces the support for lazy streams in the benchmark_with_validation_stream helper.

That helper is used to generate a benchmark instance in which the validation stream is added by splitting the train stream. If the train stream is lazily generated, then the resulting benchmark instance will have lazily generated train and valid streams.

This behavior is fully compatible with custom splitting strategies!

This partially covers #673.

The support for lazy streams in data_incremental_benchmark will be implemented in the future as it requires a major overhaul on the Avalanche internals.

[coveralls]

Pull Request Test Coverage Report for Build 957423453

• 108 of 110 (98.18%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 79.087%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
avalanche/benchmarks/scenarios/generic_cl_scenario.py	27	29	93.1%

Totals
Change from base Build 950073130:	0.1%
Covered Lines:	9481
Relevant Lines:	11988

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💛 - Coveralls