[gputests] Don't use temporary datapath in test_torchscript

[stephenroller]

Patch description
Use of a temporary datapath was preventing caching of data, and causing both data and models to be downloaded fresh every run.

Testing steps
Ran locally

Before:

275.22s call     tests/nightly/gpu/test_torchscript.py::TestTorchScript::test_torchscript_agent
82.70s call     tests/nightly/gpu/test_torchscript.py::TestTorchScript::test_token_splitter

After:

40.03s call     tests/nightly/gpu/test_torchscript.py::TestTorchScript::test_torchscript_agent
27.79s call     tests/nightly/gpu/test_torchscript.py::TestTorchScript::test_token_splitter

We'll need to bump the cache before we are likely to see this play out in CI but that will happen naturally eventually, or sooner for other speedups.

[EricMichaelSmith]

Oy, good catch, thanks for fixing