[logging] Fix suppression of debug logs

[stephenroller]

Patch description
Fix an issue where usages of create_agent_from_model_file caused logging to bump back to INFO.

The reason is that we eventually call this line to try to fill in missing args with defaults:

ParlAI/parlai/core/agents.py

Line 368 in 91e883b

opt_parser = parser.parse_args("")

Which would get the default args and accidentally call set_log_level here:

ParlAI/parlai/core/params.py

Line 1158 in 91e883b

logging.set_log_level(self.opt.get('loglevel', 'info').upper())

As a solution, I moved setting the log level to happen in the abstract ParlaiScript class.

Testing steps
Manual testing.

[mojtaba-komeili]

Thanks!

[klshuster]

thanks for the fix!!