title: the loss of mse always be 0 when keras for topic predict
body: <p>my input is a 200 dims vector, which is generated by mean of the word2vector of all words of a article,
  my output is a 50 dims vector,which is generated by the LDA results of a article
  I want to use mse as the loss function,but the value of the loss always be 0
  my code as follows:</p>
  
  <pre><code>&lt;pre&gt;model = Sequential()
  model.add(Dense(cols*footsize, 400,init = "glorot_uniform"))
  # model.add(LeakyReLU(alpha = 0.3))
  model.add(Activation('relu'))
  model.add(Dropout(0.2))
  model.add(Dense(400, 400,init = "glorot_uniform"))
  model.add(Activation('relu'))
  model.add(Dropout(0.2))
  model.add(Dense(400, 50,init = "glorot_uniform"))
  model.add(Activation('softmax'))
  model.compile(loss='mse', optimizer='rmsprop')&lt;/pre&gt;
  </code></pre>
  
  <p>the screen output as follows:
  <a href="https://i.stack.imgur.com/euVdr.png" rel="noreferrer"><img src="https://i.stack.imgur.com/euVdr.png" alt="enter image description here"></a></p>
  
  <p>who can tell me why ,thanks!</p>