Gromov barycenter example bug

[ncassereau]

Describe the bug

The example no longer runs :(
I'll suggest a PR today.

To Reproduce

Steps to reproduce the behavior:

1. Run examples/gromov/plot_gromov_wasserstein.py

Unexpected failing examples:
/home/circleci/project/examples/gromov/plot_gromov_barycenter.py failed leaving traceback:
Traceback (most recent call last):
  File "/home/circleci/project/examples/gromov/plot_gromov_barycenter.py", line 113, in <module>
    xs = np.array([np.array(xs[0]), np.array(xs[1]),
ValueError: setting an array element with a sequence. The requested array has an inhomogeneous shape after 1 dimensions. The detected shape was (4,) + inhomogeneous part.

Expected behavior

It should run smoothly.