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

[huxuan]

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:

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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.

[snarayan21]

Just the one small edit I made, other than that lgtm and will run CI on this. Thanks for the contribution @huxuan !!

[snarayan21]

BTW, it seems that pre-commit run -a and pytest -vv -s . does not work well on the latest main branch.

@huxuan Mind elaborating on what went wrong here?

[snarayan21]

lgtm!