don't add extra shuffle in DataLoader2 if one is present
Without this, `DataLoader2` will just add an `Shuffler` to the end of the datapipe if `shuffle=True`:
```py
from torch.utils.data.dataloader_experimental import DataLoader2
from torchdata.datapipes.iter import IterableWrapper, IterDataPipe, Shuffler
class Sorter(IterDataPipe):
def __init__(self, datapipe):
self.datapipe = datapipe
def __iter__(self):
return iter(sorted(self.datapipe))
data = list(range(1000))
dp = IterableWrapper(data)
dp = Shuffler(dp).set_shuffle(False)
dp = Sorter(dp)
dl2 = DataLoader2(dp, shuffle=True, batch_size=None)
assert list(dl2) == data # fails unless you hit a lucky random seed
```
This example is somewhat non-sensical, but demonstrates we cannot simply add a `Shuffler`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/75014
Approved by: https://github.com/ejguan
