* 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

* fix test_conv2d_ngraph for grad diff, test=develop

* register NoNeedBufferVarsInference for max_pool_grad_op, test=develop

* refine error message, test=develop

* fix numpy, test=develop

* disable test conv2d_ngraph_op, test=develop
Co-authored-by: NZhang Ting <709968123@qq.com>

* add shape description in accuracy api doc test=develop

* fix details test=develop

* fix details test=develop

* Fix example in log_loss doc, test=develop

* Use none for unknown dim, test=develop, test=document_fix

* test=develop fix generate_proposal_labesl op

* convert dtype of vars for range op, test=develop

* update error information about batch_norm_grad

* update bn,test=develop

fix unittest for spp op

* move private weight properties to public ones
test=develop

* revert changes to FC
test=develop

* fix unittest
test=develop

* fix unittest
test=develop

* fix coverage
test=develop

* fix merged dev
test=develop

* bug fix
test=develop

* update doc, test=develop

* fix related unittests, test=develop

* fix str incompatible error, test=develop

* add is_empty_grad_op for OpTest, test=develop

 fix the doc of scatter

* expost fluid.dygraph.TracedLayer apis, test=develop

* polish doc, test=develop

* follow comments, test=develop, test=document_fix

* follow comments, test=develop

* remove save_inference_model return value, test=develop

* enhance checking validity of apply_gradient test=develop

* refactor code test=develop

* stash

* add adamOptimizer test=develop

* add adamOptimizer test=develop

* update layers used in mnist dygraph model, test=develop

* fix import issue, test=develop

* add dygraph utils, test=develop

* add unittest, test=develop

* test=develop add fp64 check_grad for conv2d, conv3d