Add fixture to cleanup dataloader after each test runs

[oliverholworthy]

Adds a fixture to cleanup dataloader after each test runs. This ensures that if a test using the Merlin Dataloader only partially consumes a dataloader instance (and isn't using it as a context manager (with statement), or calling stop explicitly). Then we automatically clean it up by calling stop on it (it's ok if we call stop more than once).

Related:

• Add fixture to cleanup dataloader after each test runs NVTabular#1852
• Call .stop() on dataloader in test_dataloader_schema NVTabular#1848

Background

In the scenario where we have a partially consumed dataloader in a test. When combined with something like pytest-cov which scans subprocesses, this can result in the dataloader background thread hanging around and onto resources that conflict with subsequent tests. In NVTabular this resulted in subsequent tests of operators to take significantly longer than usual (>5 hours instead of ~20 mins)

[github-actions]

Documentation preview

https://nvidia-merlin.github.io/dataloader/review/pr-156