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.

Fix the docstring of 'filter groups' in img_conv_layer.

* Rename Tensor::CopyFrom to Tensor::Copy

* Fix CI

* Fix compile

Update design of dist train refactor

Error Clip Design Doc

* Remove unused LoDTensor methods

* Update

update Openblas benchmark

add back priority

fix GetDims bug

Add COWPtr and its unittest

fix ports_num_for_sparse defaut value in cluster_train doc

Release the middle layer's output memory

* create tensor in recv op

* static global function to global function

fix url in cluster_train_cn.md

fix compile error in profiler.cc

Stop gradient when pool_type=='max'

add GetPlaces operator

Enhance reorder_lod_tensor_by_rank_op to support Tensor