Rename init function of distributed strategies

[annaelisalappe]

Is your feature request related to a problem? Please describe.
It's not really a problem, but the name of the init function imo is not clearly enough distinguished from the init function of the same class (and every class, really).

Describe the solution you'd like
Renaming it something like initialize_distributed_strategy(..) would make it clearer that is corresponds to the initialisation of the distributed backend, not the class itself.