DataLoader example

[fneaplle]

I think DataLoader Usage example need to be changed.(enumerate should be added)

-----Before-------
DataLoader Usage

BaseDataLoader is an iterator, to iterate through batches:

for batch_idx, (x_batch, y_batch) in data_loader:
pass

------After---------
DataLoader Usage

BaseDataLoader is an iterator, to iterate through batches:

for batch_idx, (x_batch, y_batch) in enumerate(data_loader):
pass