For the dataset.py/init_callable, you shoul also catch Attribute error

[lmjohns3]

From @nkundiushuti (at lmjohns3/theanets#79):

I have implemented the Largedataset class as you recommended, and the dataset method yields this error:
AttributeError: LargeDataset instance has no attribute 'len'
In my opinion, you should also catch this error in dataset.py
try:
self.iteration_size = len(samples)
except TypeError: # has no len
If you want, I can share the LargeDataset class with you, but it's a bit particular to my problem. I have added the len attribute to return the number of batches in one dataset file which is loaded from hard drive.