Examples for pytorch backend throwing error if run with GPU

In the scripts exemplify the use of PyTorch in combination with POT, the torch.Generator() is always created on the CPU, which leads to an error if a GPU is available.

To reproduce the error run the example script 'plot_sliced_wass_grad_flow_pytorch.py' with an available GPU.

This is easily fixed by changing
gen = torch.Generator()
to
gen = torch.Generator(device=device)
in lines 77 and 139 of the script.
The same has to be done in the example jupyter notebook.