Introduce constant for ID_SIZE in python code. (#2517)

python/ray/remote_function.py

@@ -6,6 +6,7 @@ import copy
 import hashlib
 import inspect
 
+import ray.ray_constants as ray_constants
 import ray.signature
 
 # Default parameters for remote functions.
@@ -37,7 +38,7 @@ def compute_function_id(function):
         pass
     # Compute the function ID.
     function_id = function_id_hash.digest()
-    assert len(function_id) == 20
+    assert len(function_id) == ray_constants.ID_SIZE
     function_id = ray.ObjectID(function_id)
 
     return function_id