[tune] Split Search from Scheduling (#2452)

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

python/ray/tune/trial.py

@@ -80,6 +80,7 @@ class Trial(object):
     def __init__(self,
                  trainable_name,
                  config=None,
+                 trial_id=None,
                  local_dir=DEFAULT_RESULTS_DIR,
                  experiment_tag="",
                  resources=None,
@@ -131,10 +132,17 @@ class Trial(object):
         self.logdir = None
         self.result_logger = None
         self.last_debug = 0
-        self.trial_id = binary_to_hex(random_string())[:8]
+        if trial_id is not None:
+            self.trial_id = trial_id
+        else:
+            self.trial_id = Trial.generate_id()
         self.error_file = None
         self.num_failures = 0
 
+    @classmethod
+    def generate_id(cls):
+        return binary_to_hex(random_string())[:8]
+
     def start(self, checkpoint_obj=None):
         """Starts this trial.