Import Error for slowfast/datasets/multigrid_helper.py

**To Reproduce**
* Follow instructions in INSTALL.md.
* Run `tools/run_net.py` following GETTING_STARTED.md
* Then, error below is raised.
```bash
Traceback (most recent call last):
File "/home/eraser/projects/SlowFast/tools/run_net.py", line 9, in
from test_net import test
File "/home/eraser/projects/SlowFast/tools/test_net.py", line 17, in
from slowfast.datasets import loader
File "/home/eraser/projects/SlowFast/slowfast/datasets/loader.py", line 13, in
from slowfast.datasets.multigrid_helper import ShortCycleBatchSampler
File "/home/eraser/projects/SlowFast/slowfast/datasets/multigrid_helper.py", line 16, in
from torch._six import int_classes as _int_classes
ModuleNotFoundError: No module named 'torch._six'
```

**Reason for the error**
* The `six` module has been removed from PyTorch >= 2.0.0
* The version statement in slowfast/datasets/multigrid_helper.py only works with PyTorch versions whose major version is 1.
* The version check statement requires updates for PyTorch 2.x or later.