Added backward.calc_gradient to backpropagate gradient from given targets to inputs.

* Add squared error layers doc

* Add doc for gru_unit

* Remove cdot which isn't supported

* Update layers.rst

* Update layers.rst (minor)

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