Pass use_experimental_api=True to mixed precision functions.

This has no functional impact since the default is currently True, but I plan on changing the default to False soon.

PiperOrigin-RevId: 367049942

official/modeling/performance.py

@@ -17,6 +17,10 @@
 import tensorflow as tf
 
 
+# TODO(b/181616568): Switch default value of `use_experimental_api` in both
+# of these functions to False.
+
+
 def configure_optimizer(optimizer,
                         use_float16=False,
                         use_graph_rewrite=False,