add prefetch_factor for multiprocessing prefetching process (#41130)
Summary:
fix https://github.com/pytorch/pytorch/issues/40604
Add parameter to Dataloader to configure the per-worker prefetch number.
Before this edit, the prefetch process always prefetch 2 * num_workers data items, this commit help us make this configurable, e.x. you can specify to prefetch 10 * num_workers data items.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/41130
Reviewed By: izdeby
Differential Revision: D22705288
Pulled By: albanD
fbshipit-source-id: 2c483fce409735fef1351eb5aa0b033f8e596561
