x_num_col_dims => num_flatten_dims

* Addign document for fluid split_lod_tensor

* Adding document for fluid merge_lod_tensor

Adding documentation for the operators: lod_tensor_to_array , array_to_lod_tensor, create_array, increment (#6807)

* Adding operator assignment

* Adding a prototype for documentation in layers

* small update to re-run Travis

* Removing file from another PR

* Small typo

* Adding documentation for the operators: lod_tensor_to_array , array_to_lod_tensor, create_array, increment

* Fixing indentation issue

* Fixed datatype of input variables

* Polish API docs for assign layer

* Polishing the API docs for concat and assign layer

* Writeup for array write layer

* Fixed the type

It is useful to reorder RNN memory block.