Improve contract performance

[PabloAndresCQ]

This PR changes the way cq.contract is called across mps.py, mps_gate.py and mps_mpo.py so that:

• Handles of library and device are always passed to them as arguments.
• When doing a contraction of a small number of tensors, the explicit contraction path is given.

A subset of these changes (in particular, those applying to vdot increased the performance of Mekena's use case on GCP).

[yapolyak]

I may not have followed the logic of creating all contraction paths closely enough, but I am sure the tests would have caught if there was a mistake anywhere there.