[tune] Support lambda functions in hyperparameters / tune rllib multiagent support (#2568)

* update

* func

* Update registry.py

* revert

python/ray/tune/__init__.py

@@ -7,9 +7,9 @@ from ray.tune.tune import run_experiments
 from ray.tune.experiment import Experiment
 from ray.tune.registry import register_env, register_trainable
 from ray.tune.trainable import Trainable
-from ray.tune.suggest import grid_search
+from ray.tune.suggest import grid_search, function
 
 __all__ = [
     "Trainable", "TuneError", "grid_search", "register_env",
-    "register_trainable", "run_experiments", "Experiment"
+    "register_trainable", "run_experiments", "Experiment", "function"
 ]

python/ray/tune/suggest/__init__.py

@@ -2,9 +2,9 @@ from ray.tune.suggest.search import SearchAlgorithm
 from ray.tune.suggest.basic_variant import BasicVariantGenerator
 from ray.tune.suggest.suggestion import SuggestionAlgorithm
 from ray.tune.suggest.hyperopt import HyperOptSearch
-from ray.tune.suggest.variant_generator import grid_search
+from ray.tune.suggest.variant_generator import grid_search, function
 
 __all__ = [
     "SearchAlgorithm", "BasicVariantGenerator", "HyperOptSearch",
-    "SuggestionAlgorithm", "grid_search"
+    "SuggestionAlgorithm", "grid_search", "function"
 ]

python/ray/tune/suggest/variant_generator.py

@@ -51,6 +51,16 @@ def grid_search(values):
     return {"grid_search": values}
 
 
+class function(object):
+    """Wraps `func` to make sure it is not expanded during resolution."""
+
+    def __init__(self, func):
+        self.func = func
+
+    def __call__(self, *args, **kwargs):
+        return self.func(*args, **kwargs)
+
+
 _STANDARD_IMPORTS = {
     "random": random,
     "np": numpy,