[tune] Unflattened lookup for ProgressReporter (#9525)

Co-authored-by: Kai Fricke <kai@anyscale.com>

python/ray/tune/utils/util.py

@@ -2,7 +2,7 @@ import copy
 import logging
 import threading
 import time
-from collections import defaultdict
+from collections import defaultdict, deque, Mapping, Sequence
 from threading import Thread
 
 import numpy as np
@@ -216,6 +216,27 @@ def flatten_dict(dt, delimiter="/"):
     return dt
 
 
+def unflattened_lookup(flat_key, lookup, delimiter="/", default=None):
+    """
+    Unflatten `flat_key` and iteratively look up in `lookup`. E.g.
+    `flat_key="a/0/b"` will try to return `lookup["a"][0]["b"]`.
+    """
+    keys = deque(flat_key.split(delimiter))
+    base = lookup
+    while keys:
+        key = keys.popleft()
+        try:
+            if isinstance(base, Mapping):
+                base = base[key]
+            elif isinstance(base, Sequence):
+                base = base[int(key)]
+            else:
+                raise KeyError()
+        except KeyError:
+            return default
+    return base
+
+
 def _to_pinnable(obj):
     """Converts obj to a form that can be pinned in object store memory.