The Dataset class itself does not turn your data into Tensors.
Do you use any transformations from torchvision (e.g. `ToTensor())?
Have a look at this code. I created a dummy Dataset, store a numpy array in it:
class MyDataset(Dataset):
    def __init__(self, a):
        self.data = a

    def __getitem__(self, index):
        return self.data[index]

    def __len__(self):
        return len(self.data)

a = np.random.randn(100, 10, 10)
dataset = MyDataset(a)
b = dataset[0]
print type(b)

>> <type 'numpy.ndarray'>