There is a known issue, see here:
A walk with fastai2 - Study Group and Online Lectures Megathread (There could be more, this was the most common one we found)
You can recreate it by calling show_batch() on any DataLoader:
AttributeError Traceback (most recent call last)
in ()
----> 1 dbunch.show_batch(max_n=9, figsize=(6,7))
6 frames
/usr/local/lib/python3.6/dist-packages/fastcore/foundation.py in getattr(self, k)
221 attr = getattr(self,self._default,None)
222 if attr is not None: return getattr(attr, k)
–> 223 raise AttributeError(k)
224 def dir(self): return custom_dir(self, self._dir() if self._xtra is None else self._dir())
225 # def getstate(self): return self.dict
AttributeError: _IterableDataset_len_called