message: remove unused loss var in momentum_test.py
  remove unused loss var in momentum_test.py in `testSparseNesterovMomentum`
diff: @@ -209,7 +209,6 @@ class MomentumOptimizerTest(test.TestCase):
  accum1_np = np.array([0.0, 0.0], dtype=dtype.as_numpy_dtype)
  var0 = variables.Variable(var0_np)
  var1 = variables.Variable(var1_np)
  - loss = 5 * var0 * var0 + 3 * var1
  mom_op = momentum_lib.MomentumOptimizer(
  learning_rate=2.0, momentum=0.9, use_nesterov=True)
  x_feed = array_ops.placeholder(dtype)
  