Symptom
The following error occurs when tf.variable is used across multiple machines and multiple GPUs: WARNING:tensorflow:Gradient is None for variable:v0/tower_0/UNET_v7/sub_pixel/Variable:0.Make sure this variable is used in loss computation
Figure 1 Distributed Tensorflow unavailable
Possible Cause
Distributed TensorFlow needs to use tf.get_variable instead of tf.variable.
Solution
Replace tf.variable in the boot file with tf.get_variable.