Assert shared_parameters is true if using MP

Dynet should give an error and refuse to run if the user is using the multiprocessing code without activating setting shared_parameters = true in the DynetParams object or the initialize call.

Locate the multiprocessing code and the DynetParams or initialize entry points mentioned in the issue, then trace where multiprocessing is enabled. Confirm the existing behavior and identify the relevant test location; the work is done when invalid multiprocessing configuration raises an error and valid shared_parameters configuration still runs.

Written by the indexing model from the issue text.