Remove rank_zero_only on DataModule prepare_data

[Queuecumber]

What does this PR do?

Fixes #7927

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[Queuecumber]

cc @awaelchli

[Queuecumber]

I was looking into adding new tests but it looks like there are already decent tests for can_prepare_data, the problem is that they never actually called prepare_data to see if it worked. We could add new tests to actually call the underlying prepare_data to cover these cases if you want.

[carmocca]

Thanks for the fix!

the problem is that they never actually called prepare_data to see if it worked. We could add new tests to actually call the underlying prepare_data to cover these cases if you want.

Yes, this would be valuable.
Also, remember to update the CHANGELOG.md 👍

[codecov]

Codecov Report

Merging #7945 (a6fd8fe) into master (42c7f27) will decrease coverage by 6%.
The diff coverage is 100%.

❗ Current head a6fd8fe differs from pull request most recent head 22da0d3. Consider uploading reports for the commit 22da0d3 to get more accurate results

@@           Coverage Diff           @@
##           master   #7945    +/-   ##
=======================================
- Coverage      92%     87%    -6%     
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  Files         200     203     +3     
  Lines       12842   13129   +287     
=======================================
- Hits        11868   11398   -470     
- Misses        974    1731   +757     

[pep8speaks]

Hello @Queuecumber! Thanks for updating this PR.

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2021-06-11 17:20:35 UTC

[Queuecumber]

OK changelog and tests updated

[awaelchli]

thanks for extending the tests, great fix. Running tests now.

[Queuecumber]

I see the failing mypy check and I'll look into it ASAP

[awaelchli]

@Queuecumber thanks for the fix
The mypy failure is unrelated to your changes