Make tensorflow be optional to allow using tensorflow-gpu

[tiangolo]

As tensorflow is listed as a dependency of this project, if tensorflow-gpu (the GPU / Nvidia version) is installed, it will be overwritten and not used. It makes this package incompatible with GPUs and only work in CPU.

The change is based on one of the possible workarounds suggested in a related TensorFlow issue: tensorflow/tensorflow#7166

The change is only the section with:

 extras_require={
        "tf": ["tensorflow"],
        "tf_gpu": ["tensorflow-gpu"],
    })

The rest is just auto-formatting (yapf).

[rcmalli]

Thanks, it seems a better way to handle tensorflow dependency.

[tiangolo]

Thanks! 🎉