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:

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[XiaohanZhangCMU]

LGTM

[karan6181]

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

[snarayan21]

added unit test @karan6181

[karan6181]

You can merge the PR after the CI passes.

[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().