Add max_size mode to CombinedLoader

[janpawlowskiof]

Adds max_size mode support to CombinedLoader, which works very much like max_size_cycle, but it returns Nones instead of cycling through exhausted iterables.

Discussed in #16917

Also includes added tests for NamedTuples support in CombinedLoader, which seemed to have been missing. Hopefully this does not break to PR does only one thing rule.

Also: First time contributing, yay!

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[awaelchli]

There are some unrelated tests failing in the CI (red crosses). Don't worry about that, this will be resolved soon.