[Data] Fix progress bar initial value

[raulchen]

Why are these changes needed?

This is a bug introduced by #36679.
I thought position was the bar position (initial progress). But it is actually the line position to print the bar. Rename the variable to "progress", and default initial value to 0.

Related issue number

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[pcmoritz]

Thanks for fixing this, I'll quickly try it out before merging :)

[raulchen]

@pcmoritz I've tried your snippet locally, looked good.

[pcmoritz]

Thanks, it definitely looks better, but I'm still seeing cases where the progress bars stay uncompleted:

The full repro is with VSCode and this notebook here: https://github.com/anyscale/Made-With-ML/blob/main/notebooks/madewithml.ipynb

[raulchen]

Got it. There might be another issue. Let me merge this PR first. Will create another PR for the remaining issue(s).

[raulchen]

@pcmoritz I think that is expected. Because when you call next(ds.iter_batches)), the data is not fully consumed yet. When it's fully consumed, the progress bar will finish.