Google-colaboratory: No backend with GPU available

You need to configure the Notebook with GPU device

Click Edit->notebook settings->hardware accelerator->GPU

You'll need to try again later when a GPU is available. The message indicates that all available GPUs are in use.

The FAQ provides additional info:

How may I use GPUs and why are they sometimes unavailable?

Colaboratory is intended for interactive use. Long-running background computations, particularly on GPUs, may be stopped. Please do not use Colaboratory for cryptocurrency mining. Doing so is unsupported and may result in service unavailability. We encourage users who wish to run continuous or long-running computations through Colaboratory’s UI to use a local runtime.

There seems to be a cooldown on continuous training with GPUs. So, if you encounter the error dialog, try again later, and perhaps try to limit long-term training in subsequent sessions.