* 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

* change dygraph_to_static_output to dygraph_to_static_graph. test=develop

* Remove duplicate code. test=develop

* Follow comments from Liujie. test=develop

* change dygraph_to_static_output to dygraph_to_static_graph. test=develop

* fix smooth l1 loss op shape > 100 test=develop

* remove from white list test=develop

* add mnist to test dygraph_to_static  test=develop

* Refine adam op, test=develop

* Fuse kernels together to reduce cpu time.

* Refine paddle enforce, test=develop

* Remove some comments, test=develop

* Refine code,test=develop

* Refine cuda kernel, test=develop

* Refine code according to comments, test=develop

* Support fetch and run program in the process of dygraph_to_static_output. test=develop

* fix to_source(gast) and remove dygraph API such as Conv2D, Linear. test=develop

* Correct CPU gradients of the argsort op, form a network to test its forward and backward process, test=develop

* fix dynamic threshold error in test_argsort_op, test=develop

* dygraph support cond op
test=develop

* unittest coverage
test=develop

* fix coverage
test=develop

* fix for coverage
test=develop

* refine TypeError msg
test=develop

* remove restrict
test=develop

* support cond in clone, test=develop

* refine code, test=develop

* refine code, test=develop

* follow comments, test=develop

* refine code, test=develop

1. Considering functions, I have to change the node type from single value to a set. Because python function is allowed to return different types. The set represent all possible types

2. I added scope_name and scope_type for AstVarScope, because in python functions, variable may have different scope. For example:

```
a = 3
def foo(b):
    a = 9
    return a + b
```

the `a` in `foo` is different to the `a` out of `foo`. Similar to class field. The scope_name will help me to know the function name when static analysis finds a `return` sentence.

* dygraph support while_loop op
test=develop

* refine assign
test=develop

* register fp16 for assign op, test=develop

* add op test for fp16, test=develop

* ngraph op test skip check grad ci, test=develop

As the title

* Add two types of Metric Calculator: MultiTaskCalculator & CmatchRankCalculator.
* Add a config for DynamicAdjustChannelNum function to denote whether we will discard the remaining instances when they are not be distributed evenly.
* Remove CPU code in Pull/PushSparse and we will add it back when testing it fully.
* Fix some known issues: such as copying persistable vars after one epoch running.

* add partial_sum_op, test=develop

* modify the Paddle Error Message, test=develop

* modify the Paddle Error Message, test=develop

* modify the bug for python3, test=develop

* modify the ut for ci, test=develop

* mv to contrib, test=develop

* use check_variable_and_dtype, test=develop

* fix ci, test=develop

* fix conflict, test=dvelop

* add partial concat, test=develop

* fix the conflict, test=develop

* fix the error, test=develop

* rm SSE4, test=develop

* add partial_concat, test=develop

* fix the grids and blocks, test=develop

* fix the Paddle_Enforce, test=develop

* fix the doc of op, test=develop

* fix the doc, test=develop

* fix the doc of the op, test=develop

* replace -1 with None, test=develop

* add is_control_flow_if  test=develop

* support dygraph to static graph for simple case.

* add test for dygraph API recognition. test=develop

* support basic to_variable api. test=develop

* update dict: dygraph_class_to_static_api

* add all tests of dygraph api. test=develop

* use gast/astor instead of ast/codegen for the compatibility of PY2 and PY3. test=develop

* add arg 'num_flatten_dims' for fc ast node. test=develop

* Modify names of class by Camel-Case.

test=develop

fix the doc in elementwise ops，add the override function in the doc

* add sync communicator and implement