Various fixes and improvements

[lrzpellegrini]

The following fixes have been ported from PR #1397. Here are taken as stand-alone as they weren't strictly related to the scope of the PR.

This PR:

• Fixes Core50 benchmark with ICARL strategy #774
• Allows EwC to accept num_workers (supersedes support num_workers in EWC #1310)
• Allows W&B to log the weights checkpoint (supersedes [WIP] Fix W&B Artifacts Integration and minor errors #706)
• Prevents the creation of a unit test file in the main folder

[coveralls]

Pull Request Test Coverage Report for Build 5600673849

• 19 of 70 (27.14%) changed or added relevant lines in 7 files are covered.
• 2 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.08%) to 72.765%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tests/test_core50.py	0	3	0.0%
avalanche/evaluation/metrics/checkpoint.py	5	14	35.71%
avalanche/benchmarks/classic/core50.py	0	11	0.0%
avalanche/logging/wandb_logger.py	5	33	15.15%

Files with Coverage Reduction	New Missed Lines	%
avalanche/benchmarks/classic/core50.py	1	32.0%
avalanche/evaluation/metrics/checkpoint.py	1	51.35%

Totals
Change from base Build 5553734044:	-0.08%
Covered Lines:	16709
Relevant Lines:	22963

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💛 - Coveralls