refine unittest for fp64

Author: zhangbo9674

python/paddle/fluid/tests/unittests/test_gradient_clip.py

@@ -527,7 +527,6 @@ def test_gradient_clip(self):
  [16, 5], min=-10, max=10).astype('float64')
  linear = fluid.dygraph.Linear(5, 5, dtype="float64")
  out = linear(fluid.dygraph.to_variable(inputs))
- out = linear(fluid.dygraph.to_variable(inputs))
  loss = fluid.layers.reduce_mean(out)
  loss.backward()
  # before clip