Merge branch 'develop' of https://github.com/PaddlePaddle/Paddle into dropout.

update the how to write op doc.

fix clang build and run error

Op refine for making build system more automatic

update the doc for how to write the operators.

Add cosine similarity operator.

make dim int to int64_t

update the doc of softmax_op.

Move two tests form `op_registry_test` to `operator_test`

add repr for PyDataProvider2.InputType.

add how to use eigen cn doc

Make attribute support for std::vector<std::pair<int, int>>

Fix compile warnings in lookup_op

1. TEST(ProtoMaker, DuplicatedAttr)
2. TEST(ProtoMaker, DuplicatedInOut)

Enhance the unit test framework to explicitly test whether the operator correctly handles gradients for multiple inputs.

Fix conv3d DeConv3d (bias shape)

Invoke check_grad many times for no_grad_set

add missing protostr for sub_nested_seq_layer.