Fix metrics decode error on non-local platform (#2152)

src/sdk/pynni/nni/platform/local.py

@@ -63,9 +63,8 @@ def get_next_parameter():
 
 def send_metric(string):
     if _nni_platform != 'local':
-        data = (string).encode('utf8')
-        assert len(data) < 1000000, 'Metric too long'
-        print('NNISDK_ME%s' % (data), flush=True)
+        assert len(string) < 1000000, 'Metric too long'
+        print("NNISDK_MEb'%s'" % (string), flush=True)
     else:
         data = (string + '\n').encode('utf8')
         assert len(data) < 1000000, 'Metric too long'

src/sdk/pynni/nni/trial.py

@@ -125,8 +125,9 @@ def report_final_result(metric):
 
     Parameters
     ----------
-    metric:
-        serializable object.
+    metric: serializable object
+        Usually (for built-in tuners to work), it should be a number, or
+        a dict with key "default" (a number), and any other extra keys.
     """
     assert _params or trial_env_vars.NNI_PLATFORM is None, \
         'nni.get_next_parameter() needs to be called before report_final_result'