Add backward.ensure_trial_working_dir

Why:

Database filled with previous version of Oríon will have trials with no
exp_working_dir. We must make sure that these trials when suggested will
have their exp_working_dir set properly before being passed to the
function to optimize.

src/orion/client/experiment.py

@@ -15,6 +15,7 @@
 import orion.core
 import orion.core.utils.format_trials as format_trials
 from orion.core.io.database import DuplicateKeyError
+from orion.core.utils import backward
 from orion.core.utils.exceptions import (
     BrokenExperiment,
     CompletedExperiment,
@@ -842,6 +843,7 @@ def _optimize(
                 with self.suggest(
                     pool_size=pool_size, timeout=reservation_timeout
                 ) as trial:
+                    backward.ensure_trial_working_dir(self, trial)
 
                     kwargs.update(flatten(trial.params))
 

src/orion/core/utils/backward.py

@@ -184,3 +184,9 @@ def algo_observe(algo, trials, results):
             trial.results.append(Trial.Result(name=name, type=name, value=trial_result))
 
     algo.observe(trials)
+
+
+def ensure_trial_working_dir(experiment, trial):
+    """If the trial's exp working dir is not set, set it to current experiment's working dir."""
+    if not trial.exp_working_dir:
+        trial.exp_working_dir = experiment.working_dir