[tune] Add encoder for PBT (#5599)

* Add encoder

* Apply suggestions from code review

python/ray/tune/schedulers/pbt.py

@@ -13,6 +13,7 @@ import shutil
 
 from ray.tune.error import TuneError
 from ray.tune.result import TRAINING_ITERATION
+from ray.tune.logger import _SafeFallbackEncoder
 from ray.tune.schedulers import FIFOScheduler, TrialScheduler
 from ray.tune.suggest.variant_generator import format_vars
 from ray.tune.trial import Trial, Checkpoint
@@ -276,13 +277,13 @@ class PopulationBasedTraining(FIFOScheduler):
         ]
         # Log to global file.
         with open(os.path.join(trial.local_dir, "pbt_global.txt"), "a+") as f:
-            f.write(json.dumps(policy) + "\n")
+            print(json.dumps(policy, cls=_SafeFallbackEncoder), file=f)
         # Overwrite state in target trial from trial_to_clone.
         if os.path.exists(trial_to_clone_path):
             shutil.copyfile(trial_to_clone_path, trial_path)
         # Log new exploit in target trial log.
         with open(trial_path, "a+") as f:
-            f.write(json.dumps(policy) + "\n")
+            f.write(json.dumps(policy, cls=_SafeFallbackEncoder) + "\n")
 
     def _exploit(self, trial_executor, trial, trial_to_clone):
         """Transfers perturbed state from trial_to_clone -> trial.