Import Error for slowfast/datasets/multigrid_helper.py

[sirzzang]

To Reproduce

• Follow instructions in INSTALL.md.
• Run tools/run_net.py following GETTING_STARTED.md
• Then, error below is raised.

Traceback (most recent call last):
  File "/home/eraser/projects/SlowFast/tools/run_net.py", line 9, in <module>
    from test_net import test
  File "/home/eraser/projects/SlowFast/tools/test_net.py", line 17, in <module>
    from slowfast.datasets import loader
  File "/home/eraser/projects/SlowFast/slowfast/datasets/loader.py", line 13, in <module>
    from slowfast.datasets.multigrid_helper import ShortCycleBatchSampler
  File "/home/eraser/projects/SlowFast/slowfast/datasets/multigrid_helper.py", line 16, in <module>
    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.