message: MomentumOptimizer has two required args which causes error in optimize_loss.
  optimize_loss function tries to initialize Optimizer like...
  opt = OPTIMIZER_CLS_NAMES[optimizer](learning_rate=lr)
  This does not work for MomentumOptimizer which also has required arg
  of 'momentum'.
diff: @@ -41,7 +41,7 @@ OPTIMIZER_CLS_NAMES = {
  "Adagrad": train.AdagradOptimizer,
  "Adam": train.AdamOptimizer,
  "Ftrl": train.FtrlOptimizer,
  - "Momentum": train.MomentumOptimizer,
  + "Momentum": lambda lr: train.MomentumOptimizer(lr, momentum=0.9),
  "RMSProp": train.RMSPropOptimizer,
  "SGD": train.GradientDescentOptimizer,
  }
  