Created by: JiayiFeng

1. Register assign_value_op an empty grad_op, so we are able to use it in a normal network configuration.

2. Refine assign layer. Give assign layer's second parameter 'output' a default value: None. If it is None, the output variable will be created inside the layer.

3. Refine CustomReader. Add a member variable 'scope_' to it. The 'scope_' will act as the global scope of preprocessing, making it possible to save something cross batches.

4. Add a new attribute named 'startuo_seed' to RandomCropOp. If the input 'Seed' is not initialized, the 'startup_seed' will be used to replace it.

5. Make buffers of DoubleBufferReader and open_files bigger.