fix: expand user path for Writer's output directory.

Description of changes:

When setting the out argument of Writer (e.g. MDSWriter) to something like ~/awesome-streamdataset, it will create a directory in current workdir named with "~" which is quite strange.

Issue #, if available:

Merge Checklist:

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General

[x] 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

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[ ] I have added tests that prove my fix is effective or that my feature works (if appropriate).
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BTW, it seems that pre-commit run -a and pytest -vv -s . does not work well on the latest main branch.

mosaicml / streaming