Fix runtime error induced by #18

[cpacker]

https://github.com/deepmind/gqn-datasets/pull/18 creates a runtime NameError (pops up when you run the demo code). This is a small patch to fix it.

Traceback (most recent call last):
  File "demo_code.py", line 5, in <module>
    data_reader = DataReader(dataset='rooms_free_camera_no_object_rotations', context_size=5, root=root_path)
  File "data_reader.py", line 182, in __init__
    for _ in range(num_threads)]
  File "data_reader.py", line 182, in <listcomp>
    for _ in range(num_threads)]
  File "data_reader.py", line 219, in _make_read_op
    indices = self._get_randomized_indices(seed)
NameError: name 'seed' is not defined

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