run_example_debug on Mac OS doesn't save videos to the right directory

[justinvyu]

In debug mode, saves to "softlearning/videos" instead of under the user's home directory "~/ray_results/...". Works fine using run_example_local.

[hartikainen]

Noting myself that this is due to the change in tune's debug mode handling. A fix is to add os.chdir(self.logdir) in the beginning of the ExperimentRunner setup.