Fix for mono input

Signed-off-by: Jael Gu <mengjia.gu@zilliz.com>

nn_fingerprint.py

@@ -115,7 +115,9 @@ class NNFingerprint(NNOperator):
             audio = numpy.vstack(frames).transpose()
         else:
             audio = numpy.hstack(frames)
+            if len(audio.shape) == 1:
                 audio = audio[None, :]
+            assert len(audio.shape) == 2
         audio = self.int2float(audio)
 
         if sr != self.params['sample_rate']: