This is weird. TensorFlow sometimes messes up when you mix CPU and GPU packages (or release and nightly packages) in the same virtual environment. Could you try doing the following steps in a fresh virtualenv? It’s basically avoiding the first install of tensorflow since you want to install tensorflow-gpu. Also, you don’t need the deepspeech package for training.
pip3 install -r <(grep -v tensorflow requirements.txt) pip3 install $(python3 util/taskcluster.py --decoder) pip3 install tensorflow-gpu==1.14.0