Update spsa.py

[rsandler00]

Currently, although SPSA attack theoretically works with both differentiable and non-differentiable classifiers, its use with the latter is limited since it expects a TF tensor output. Thus it cannot be tested w/ non-TF classifiers (e.g. random forests, scipy, etc...) which output Numpy arrays instead of TF tensors. This is because line 49 uses logits.get_shape().

To fix this, I propose to sub logits.get_shape to logits.shape since this would generalize to both Numpy arrays and TF tensors.

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[rsandler00]

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