[vec][loss] update docstring, test=doc fix #1717

Author: qingen

paddlespeech/vector/modules/loss.py

@@ -132,6 +132,9 @@ def __init__(self, Q, noise_ratio=100, Z_offset=9.5):
 
     def forward(self, output, target):
         """Forward inference
+        
+        Args:
+            output (tensor): the model output, which is the input of loss function
         """
         output = paddle.reshape(output, [-1, self.N])
         B = output.shape[0]