[tune] Split Search from Scheduling (#2452)

Introduces SearchAlgorithm concept, separate from schedulers in Tune. Moves HyperOpt under this concept.

python/ray/tune/experiment.py

@@ -79,3 +79,41 @@ class Experiment(object):
         exp.name = name
         exp.spec = spec
         return exp
+
+
+def convert_to_experiment_list(experiments):
+    """Produces a list of Experiment objects.
+
+    Converts input from dict, single experiment, or list of
+    experiments to list of experiments. If input is None,
+    will return an empty list.
+
+    Arguments:
+        experiments (Experiment | list | dict): Experiments to run.
+
+    Returns:
+        List of experiments.
+    """
+    exp_list = experiments
+
+    # Transform list if necessary
+    if experiments is None:
+        exp_list = []
+    elif isinstance(experiments, Experiment):
+        exp_list = [experiments]
+    elif type(experiments) is dict:
+        exp_list = [
+            Experiment.from_json(name, spec)
+            for name, spec in experiments.items()
+        ]
+
+    # Validate exp_list
+    if (type(exp_list) is list
+            and all(isinstance(exp, Experiment) for exp in exp_list)):
+        if len(exp_list) > 1:
+            print("Warning: All experiments will be"
+                  " using the same Search Algorithm.")
+    else:
+        raise TuneError("Invalid argument: {}".format(experiments))
+
+    return exp_list