We need this operator to assign value to a tensor and the values are stored in the program so that they can be used independent of python.

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

* Polish API docs for assign layer

* Polishing the API docs for concat and assign layer

* Simplize system_allocator and fix GPU_INFO

* Remove main_program/startup_program

* Fix optimizer

* Fix CI

* Follow comments

* Make cast op support bool

Also add `elemwise_sub/mul/abs/clip` layers

* Make fuild.layers as a module

* Move layers as a module

* Split layers.py into layers module

* Fix CI

* Fix CI