* add LayerList insert and extend
test=develop

* add index range check
test=develop

* add sample codes
test=develop

* refine sample code
test=develop

* fix init_gflags with 'python -c', test=develop

* enhance error msg related Tracer, test=develop

* refine err msg, test=develop

* follow comments, test=develop

* API(to_variable/save_dygraph) error message enhancement

error message enhancement for fusion_seqpool_concat_op

error message enhancement for py_func op. 

* Involves AfsAPI to resolve slow downloading.
* Mainly used in PaddleBox

* error message enhancement for Conv2DTranspose, test=develop

* fix error_message, test=develop

* solve the conflict of ops with the same name. test=develop

* error message of NCE API enhancement

Fix the bug of matmul test_case

* fix no_grad signature
test=develop

* check func name instead of doc
test=develop

* API(program_guard) error message enhancement test=develop

* fix Cycle import test=develop

* polish code according comment test=develop

* refine fusion_transpose_flatten_concat_op log
test=develop

* fix ci error
test=develop

* Layer add training attr, add Dropout Layer

* add unit test for Dropout
test=develop

* minor fix
test=develop

* add missing args
test=develop

* support global flag in dropout, add docs
test=develop

* minor fix
test=develop

* minor fix
test=develop

* refine code comments
test=develop

* refine Dropout
test=develop

* fix ut
test=develop

* arg expansion
test=develop

* sample code update
test=develop

* prop -> p
test=develop

* fix ut
test=develop

* minor check fix
test=develop

C++ error message enhancement.