QCNN backend issues

[BrunoLiegiBastonLiegi]

The QCNN doesn't allow to pass a backend, but always makes use of the GlobalBackend

qibo/src/qibo/models/qcnn.py

Line 12 in 0d86ec5

class QuantumCNN:

.

In the tests, indeed, only the GlobalBackend is tested which, by chance, was working (I'm guessing the numpy one was used).
Some quick test I ran showed that the TensorflowBackend might have problems with it, and probably other backends as well.