* set esp as 1e-6 to solve elu unitest fail,test=develop

* add special way to add distribute vars， Update Pyramid hash op

* fix Variable's gradient api in framework.py, test=develop

* remove namescope, test=develop

* Fix the global_step & continuous applying error in EMA

test=develop

* Fix for step 0 & add unit test, test=develop

* fix grad clip， clip op belongs to Backward op when running in Parameter Server mode.

* add erf op and python interface.

* add fp16 support for erf op.

* add unitests for erf op and its python interface.

enhanced ops: conv2d, conv3d
elementwise_pow: change to a reasonable shape

* support elu activation double grad,test=develop

* delete the code commit in .cc,test=develop

* fix relu test unpass, test=develop

* add elu double grad kernel and unit test

* add caculate dX in elu double grad functor, test=develop

* update the commit code,test=develop

* add gelu plugin

* align trt bert with gpu

* add support for fused fc with relu,

* add unittest for bert trt

* add dygraph LayerList

* add unittest
test=develop

* add newline
test=develop

* typo fix
test=develop

* fix dim=[] bug in reduce ops

enhanced ops: unsqueeze, squeeze2, strided_slice, unsqueeze,
unsqueeze2, var_conv_2d, spectral_norm, slice, match_matrix_tensor,
nce, pad, pad_constant_like, filter_by_instag

affine_channel, affine_grid, bilinear_interp, bilinear_tensor_product,
clip, crop, crop_tensor, cumsum, data_norm, expand, expand_as

* add distributed_strategy

flatten, flatten2, gather, gather_nd, group_norm, gru,
gru_unit, reduce_max, reduce_min, reduce_mean, reduce_sum,
reshape, scatter_nd_add, sequence_expand, sequence_expand_as,
sequence_pad

* append optimize op in the grad block of current block if current block is in control flow. test=develop

* add conditional grad op when optimizer used in control flow. test=develop

* add comment and modify typo. test=develop

* fix append_backward to support control flow. test=develop

* add test. test=develop

* fix copy_var_to_parent_block and conditional_block_grad. test=develop

* fix bug: revert to append conditional_block_grad vars to sub grad block. test=develop

* fix bug: revert to assign var to parent block even if var already is in parent block

* fix bug: consider outputs is empty. test=develop

* move _rename_grad_ out. test=develop

* modify code according to reviews from Huihuang. test=develop

* modify code according to reviews from Jinle. test=develop

* register int/int64_t/float16 in  pow/square kernel,test=develop

* add abs/square/exp type,test=develop