fix pl_worker_init_fn for pytorch 1.9

## 🐛 Bug

Investigate a test.

### To Reproduce

Run this test:

```
FAILED tests/trainer/test_dataloaders.py::test_missing_worker_init_fn - asser...
= 1 failed, 1422 passed, 263 skipped, 3 xfailed, 2760 warnings in 435.18s (0:07:15) =
```

### Expected behavior

obvious. 

### Environment

PyTorch 1.9 (nightly)

