Multithread batch sampler for `PatchInferer`

[drbeh]

Fixes #6138

Description

This PR makes the batch sampling of PatchInferer multi-threaded so that WSI inference (using WSI splitters) can leverage this to enhance performance.

Types of changes

• [x] Non-breaking change (fix or new feature that would not break existing functionality).
• [x] New tests added to cover the changes.
• [x] Integration tests passed locally by running ./runtests.sh -f -u --net --coverage.
• [x] Quick tests passed locally by running ./runtests.sh --quick --unittests --disttests.
• [x] In-line docstrings updated.

[drbeh]

@ericspod any more comments here? Otherwise, we can merge it. Thanks

[wyli]

/build