Make users can access parameters of layers and their gradients.

Add sequence erase op

Async GRPC sendrecv

fix ds2 issue

auto set openblas env

Add edit distance operator

* init data_type_transform

* split data_layout_transform

* tmp rm data_transform_test

* change device_data_transform to data_device_transform

* clean code

* clean code

Polish GetPlacesOp

Enhence shrink_rnn_memory_op.

feature/parallel_gpu

Calculating gradients for partial graph

* "init use all default devices"

* "fix init test"

* add lod tensor ToAbsOffset test

* add share lod to topk op and softmax op

Refine while grad op

Add grad_op_maker for sequence_pool

Add script to plot learning curve

Added backward.calc_gradient to backpropagate gradient from given targets to inputs.

* test dist word2vec

* multiple trainers work

* Add Seq2BatchFunctor, which will be used in WarpCTCOp.

* Implement WrapCTCFunctor and WrapCTCKernel.

* Add unittest of warpctc_op.

* Modify the check_output inferface in python unittest framework to allow check a subset of outputs.

* Use absolute offset lod in warpctc_op and related functors.

* Refine the comments of warpctc_op.

* The new python unittest supports checking a subset of the outputs, so revoke the previous change.

* Rename the transform from LoDTensor to Tensor with shape [max_sequence_length, num_sequences, sequence_width] to PaddingSequenceFunctor.

* Update to the newest codes.

* Rename the PaddingSequenceFunctor to PaddingLoDTensorFunctor and remove the computation of dimensions out of the functos.