Bug: TruncableJoblibStepSaver needs a small rework

[vincent-antaki]

The save_step/load_step methods of this saver define two cases for each sub-step: one if the should_save method returns True, and one if it returns False. The later crashes on load_step calls by using the TruncableStep getitem method.

I believe we should find a way to get rid of the second case. I tried to do so quickly, but got some error from the ResumablePipeline tests. This should be investigated at the same time as issue #470.

[stale[bot]]

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