contrib/eagerpython/datasets: Resource naming workaround.

tensorflow/contrib/eager/python/datasets_test.py was failing on GPU
because two tests - testTensorsPlacedOnDevice() and
testTensorsExplicitPrefetchToDevice() we're creating
FunctionBufferResources with the same shared_name, leading to
unintentional interference.

This change will make the tests pass and allow the use of
tf.contrib.eager.Iterator and
tf.data.Dataset.apply(prefetching_ops.prefetch_to_device)
in the same process without interference.

However, a more appropriate fix would probably be to use
anonymous function buffering resources (similar to
AnonymousIteratorHandle) when eager execution is enabled,
doing away with sharing by name.