test=develop

* no longer need to define all embedding layers (no one less) of all slots in each program. make trainer_param repeated in ps.proto.
* add find_distributed_lookup_table_grads instead of hard code GRAD
* support embedding stop gradient. push sparse has error before fix this.* 
* fix fill sparse, skip slots which do not have embedding. each slot's embedding in a sparse table should be used in all training programs before fix this.
* fix pull sparse, skip slots which do not have embedding.
* fix collect feasign label info, skip slots which do not have embedding.
* support when there are multi sparse tables in one or multi training programs, each program can pull/push its own related sparse tables instead of all sparse tables.
* test=develop

test=develop

* All elements in attr(shape) of crop_tensor can be -1, test=develop, test=document_preview

* fix the bug that attr(offsets) should be initialized, test=develop

* test=develop

* test=develop

* test=develop

* test=develop

* make search_compute support avx only

* clean search_compute.h

* rename sse_axpy to avx_axpy

test=develop

* update CMakeLists.txt

test=develop

* jit/trace 1st version, test=develop

* add more unittests, test=develop

* refine Categorical and MultivariateNormalDiag en doc test=develop, test=document_fix

* refine Categorical and MultivariateNormalDiag en doc test=develop, test=document_fix

* mv sampcd_processor.py to tools

test=develop test=document_fix

* update example script

test=develop test=document_fix

* update the infer shape of matmul,  test=release/1.6

* add unittests of matmul, test=release/1.6

* change func names, test=develop

* add support for __get_item__ of Variable test=develop

* add cuda place and remove fluid.layers.data for test of python API, test=develop

* add cuda place and remove fluid.layers.data for test of python API, test=develop

* modified batch size for Python API test, test=develop

* add dlpack include in third_party. test=develop

* test=develop, communicator_is_sgd_optimizer flags

* add check nan / inf in downpour worker during training
* test=develop

* fix fetch handler error with pslib
* fix distributed lookup table op with 1 pserver

* move pool2d to add_quant_dequant_pass, test=develop

* fix elementwise_floordiv_op and elementwise_mod_op, test=develop

* fix API.spec, test=develop

* fix API.spec, test=develop

test=develop

* Support fp16 in fused_elemwise_activation_op.
* Fix unit testing in ONLY-CPU mode.