flexible hypop doesn't remove kwargs at default

[AlessandroLovo]

flexible_hypop.py directly calls

self.trainer._run(**run_args)

which means that the check and removal of arguments at their default value is not performed.
This is a minor bug, which simply affects how the runs.json file looks.

Possible solutions are:

1. to perform the argument check in the _run function of the Trainer class
2. Manually remove args at default in flexible hypop
3. Do nothing

[AlessandroLovo]

Went with option 1