Replace memory batch size with a fraction of the total batch size

[wistuba]

New attempt of #217

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[github-actions]

Coverage report

The coverage rate went from 85.68% to 85.68% ⬇️

72.72% of new lines are covered.

Diff Coverage details (click to unfold)

src/renate/cli/parsing_functions.py

33.33% of new lines are covered (79.51% of the complete file).
Missing lines: 96, 111

src/renate/updaters/avalanche/learner.py

100% of new lines are covered (97.36% of the complete file).

src/renate/defaults.py

100% of new lines are covered (99.01% of the complete file).

src/renate/updaters/learner.py

83.33% of new lines are covered (96.8% of the complete file).
Missing lines: 459

[lballes]

Not sure why it works now, but LGTM.

In this version, offline ER still uses only batch_size in the initial update rather than batch_size + memory_batch_size, but I can adapt this in a separate PR.