[tune] Usability errors PBT (#5972)

* update with upstream master

* check for function args in hyperparam_mutations pbt

* fix style for pbt

* remove_checkpoint

* Update pbt.py

* Update pbt.py

* fix

Co-authored-by: Richard Liaw <rliaw@berkeley.edu>

python/ray/tune/schedulers/pbt.py

@@ -183,6 +183,11 @@ class PopulationBasedTraining(FIFOScheduler):
                  resample_probability=0.25,
                  custom_explore_fn=None,
                  log_config=True):
+        for value in hyperparam_mutations.values():
+            if not (isinstance(value, list) or callable(value)):
+                raise TypeError("`hyperparam_mutation` values must be either "
+                                "a List or callable.")
+
         if not hyperparam_mutations and not custom_explore_fn:
             raise TuneError(
                 "You must specify at least one of `hyperparam_mutations` or "