Shuffle doesn't work

Hi all!

Below an examle of code:
```
from merlin.loader.torch import Loader
from merlin.io import Dataset


train_ds = Dataset('train.parquet')
train_loader = Loader(train_ds, batch_size=65536, shuffle=True)

for batch in train_loader:
    print(batch)
```

After running I got following:
```
TypeError: sample() got an unexpected keyword argument 'keep_index'
```
