TensorFlow module error causes tests to fail

[j-s-ashley]

Lines 126 and 149 of the test_archs.py file are causing CI to fail with the following error:

'keras._tf_keras.keras.backend' has no attribute 'function'

The lines in the test file

126 kf = K.function(inputs=efn.inputs, outputs=efn.latent)

149 kf = K.function(inputs=pfn.inputs, outputs=pfn.latent)

refer to the import from line 9: from tensorflow.keras import backend as K