+ Refine code structure and move code related with control flow `if` into `ifelse_transformer.py`
+ Merge code of `ast_utils.py`  into `utils.py` 

As the title.

* fix add grad bug test=develop

* update style test=develop

* refine grad maker, test=develop

* refactor tracer stage 1, test=develop

* merge develop to solve conflict third times, test=develop

* fix API param bug of recompute.backward(), test=develop

add lookup_table_dequant_op

* support basic tensor.shape. 

* Support tensor.shape with dependencies. 

* polish detail implement of data loader, test=develop

* solve coverage ci problem, test=develop

In elementwise_min api doc has the error, all the api just use the elementwise_max op, we just correct the doc.

1. copy.deepcopy in NameVisitor should be changed to copy.copy to make hash or set work
2. read_context should be type of gast.Load()/gast.AugLoad(), not gast.Load/gast.AugLoad

Add basic support for while in translating dygraph to static

1. Analysis the variable liveness in class NameVisitor
2. Replace while key word using while_loop API

* Added an option to use external FP32 model in QAT comparison test

* Modify the way of inserting newly defined func_nodes test=develop

* add recorded cuda memory apis, fix typo, test=develop

* add more ut, test=develop

* follow comments, test=develop

* fix py35 incompatible issues, test=develop

* add fluid.device_guard to specify the device type for Op

* functional dygraph enable/disable
test=develop

* use context manager instead
test=develop

* refine sample code
test=develop

* rename api & expose to fluid
test=develop

* fix sample code
test=develop

* test=develop, optimize distributedstrategy

test=develop

Fix the big shape op test, elementwise_mul, elementwise_div, mul op
Fix the precision op test, mul op

* polish the profiler doc

* update ScopeBufferedSSAGraphExecutor&AsyncSSAGraphExecutor&ThreadedSSAGraphExecutor&FastThreadedSSAGraphExecutor&ParallelSSAGraphExecutor&ParallelExecutor for fetching unmerged results.

* add the unit test for fetch_unmerged.

* update ut for multi-card and multi-cpu.

* add the error message and the user suggestion in FetchOpHandle. test=develop

* fix basic gru lstm parameter attr bug; test=develop

* fix bias attr bug; test=develop

* add basic lstm gru name unitest; test=develop

* upgrade fp32 to fp64, remove no_grad_set, enlarge input shape for hsigmoid unittest test=develop

* revert "remove no_grad_set" test=develop

* add lodtensor share memory & serialization, test=develop

* fix windows compile error, test=develop

* deal vartype pickle & fix unittest matching error message, test=develop

* update timeout variable name, test=develop

* refactor memory map implement, test=develop

* clear mmap file discripter when exit unexpectedly, test=develop

* remove the child process fd in advance, test=develop

* remove mmap fds after Queue.put in child process, test=develop

* add hard unittests for register exit func, test=develop

* fix python2 compatibility problem in unittest, test=develop

* fix exception unittest error, test=develop

* polish code based review comment, test=develop

* user can call dataset.set_download_cmd to set its customized download cmd
* add UT to cover this scenario

* test ResNet decorated by dygraph_to_static_output in static mode. test=develop

* Follow comments and change dygraph_to_static_output to dygraph_to_static_graph.test=develop

* Add the codegen and auto fusion for sum Op  in fusion group