Revert "[data.datamodule] Experiment with fork/spawn setup"

This reverts commit 20dfcdb4.

src/ptbench/data/datamodule.py

@@ -584,7 +584,7 @@ class ConcatDataModule(lightning.LightningDataModule):
             num_workers = value or multiprocessing.cpu_count()
         self._dataloader_multiproc["num_workers"] = num_workers
 
-        if num_workers > 0:  # and sys.platform == "darwin":
+        if num_workers > 0 and sys.platform == "darwin":
             self._dataloader_multiproc[
                 "multiprocessing_context"
             ] = multiprocessing.get_context("spawn")