Improve local temp directory error when only `remote` is specified

[snarayan21]

Description of changes:

Improves the local temp directory error when only a remote path is specified. We want to make sure that if local is unspecified, we still create an entirely new temporary directory to hold the shard files, but if a user wants to reuse their locally cached dataset, they should explicitly specify the local argument.

Issue #, if available:

Merge Checklist:

Put an x without space in the boxes that apply. If you are unsure about any checklist, please don't hesitate to ask. We are here to help! This is simply a reminder of what we are going to look for before merging your pull request.

General

• [ ] I have read the contributor guidelines
• [ ] This is a documentation change or typo fix. If so, skip the rest of this checklist.
• [ ] I certify that the changes I am introducing will be backward compatible, and I have discussed concerns about this, if any, with the MosaicML team.
• [ ] I have updated any necessary documentation, including README and API docs (if appropriate).

Tests

• [ ] I ran pre-commit on my change. (check out the pre-commit section of prerequisites)
• [ ] I have added tests that prove my fix is effective or that my feature works (if appropriate).
• [ ] I ran the tests locally to make sure it pass. (check out testing)
• [ ] I have added unit and/or integration tests as appropriate to ensure backward compatibility of the changes.

[karan6181]

Can we please add a unit test for the same? :D

[snarayan21]

added unit test @karan6181

[snarayan21]

@karan6181 @XiaohanZhangCMU I was trying to fix the test that I wrote, but I realized there's also a test_existing_local_raises_exception in test_stream.py that tests this already. Are we good with merging the PR without me adding an additional test then?

[karan6181]

@karan6181 @XiaohanZhangCMU I was trying to fix the test that I wrote, but I realized there's also a test_existing_local_raises_exception in test_stream.py that tests this already. Are we good with merging the PR without me adding an additional test then?

Yes, it's okay to merge since the unit test is already in place. To make CI pass, you might want to change the exception type from ValueError to FileExistsError in test_existing_local_raises_exception().