Created by: Shixiaowei02

update some examples of python Paddle API:

1. paddle.fluid.layers.matmul

2. paddle.fluid.layers.lod_reset 3. paddle.fluid.layers.log 4. paddle.fluid.layers.mean 5. paddle.fluid.layers.isfinite 6. paddle.fluid.layers.less_than 7. paddle.fluid.layers.merge_selected_rows

test=document_preview test=develop