Expand local_dir in Trial init (#2013)

* Fix the case where Trial logs into wrong paths when `local_dir`
argument starts with tilde (~), by expanding the `local_dir` argument
* Add test case for checking that the tilde gets expanded

python/ray/tune/trial.py

@@ -110,7 +110,7 @@ class Trial(object):
         # Trial config
         self.trainable_name = trainable_name
         self.config = config or {}
-        self.local_dir = local_dir
+        self.local_dir = os.path.expanduser(local_dir)
         self.experiment_tag = experiment_tag
         self.resources = (
             resources