Fix crash when --ml_perf flag is not specified. (#5610)

The error message was:

absl.flags._exceptions.IllegalFlagValueError: flag --ml_perf=None: ('Non-boolean argument to boolean flag', 'None')

official/recommendation/ncf_main.py

@@ -403,7 +403,7 @@ def define_ncf_flags():
           "achieved by MLPerf implementation."))
 
   flags.DEFINE_bool(
-      name="ml_perf", default=None,
+      name="ml_perf", default=False,
       help=flags_core.help_wrap(
           "If set, changes the behavior of the model slightly to match the "
           "MLPerf reference implementations here: \n"