Make OP_INSTANCE_KEY_START_NUMBER a constant.

PiperOrigin-RevId: 289990581
Change-Id: I2d35e1a9db8313d7406bfc9b7b5e2a6604af76da

tensorflow/python/distribute/cross_device_utils.py

@@ -35,6 +35,9 @@ from tensorflow.python.ops import math_ops
 from tensorflow.python.ops import nccl_ops
 
 
+OP_INSTANCE_KEY_START_NUMBER = 100
+
+
 def aggregate_gradients_using_nccl(replica_grads):
   """Aggregate gradients using nccl allreduce."""
   agg_all_g_and_v = []
@@ -253,7 +256,7 @@ class CollectiveKeys(object):
 
   def __init__(self,
                group_key_start=1,
-               op_instance_key_start=100,
+               op_instance_key_start=OP_INSTANCE_KEY_START_NUMBER,
                variable_instance_key_start=1000000):
     """Initializes the object.