Add helpers to disable progress bars globally + tests

[Wauplin]

Solve #978 (see API discussion in issue).

@sgugger Since both HF_HUB_DISABLE_PROGRESS_BARS env variable and disable_progress_bars()/enable_progress_bars() helpers are defined, there might be a conflict at some point if not correctly handled. For example a library, could call disable_progress_bars() even though the user has set HF_HUB_DISABLE_PROGRESS_BARS=0. Current solution is that disable_progress_bars() has higher priority but I'm not 100% sure it's clever.

The other solution would be the opposite: if HF_HUB_DISABLE_PROGRESS_BARS is set (either 0 or 1), then disable_progress_bars/enable_progress_bars are ignored in the code with a warning ("cannot programmatical disable progress bars as verbosity has been set by environment variable").

What is your opinion about it ? I feel that both are correct, it's more a usage preference.

EDIT: after discussion below, we went for solution 2 (environment variable has priority).

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.

[sgugger]

Thanks a lot for working on this!

I think the env variable should get priority over programmatic functions. My reasoning being that if the user goes out of their way to set it on the env level, it's really clear they want that thing all the time. But it's just my 2 cents and going the other way around also works.

[Wauplin]

I think the env variable should get priority over programmatic functions. My reasoning being that if the user goes out of their way to set it on the env level, it's really clear they want that thing all the time. But it's just my 2 cents and going the other way around also works.

Makes sense. That's what I also started to think when writing the PR text actually but didn't want to change the code before having a second opinion :D
Will make the change.

[Wauplin]

@sgugger I made the changes to give priority to env variable in commit b47378a.
Could you quickly review it so that I can merge ? Thanks in advance 🙏

[sgugger]

LGTM!