* support to modify dict and list in while_loop test=develop

* polish code test=develop

1. Add basic support for `for in range` loop
2. Move `test_dygraph_to_static_*` to `dygraph_to_static` dir and rename them
3. Add test case for dict in while_loop

Modify english document and unittest of while_loop

This PR makes assign op support LoDTensorArray and enable the loop_vars in
while_loop to support tuple or list.

1. Make while_op accept GPU conditional data
2. Add more complex test cases for while_loop API

Add basic while_loop