MKLDNN conv + elementwise_add fusion: skip connection attribute renamed. Comments about patterns added.

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MKLDNN conv + elementwise_add fusion: some refactoring: consts, function calls instead of constant values

MKLDNN conv + elementwise_add fusion: graph is corrected based on actual argument name, not formal argument name

MKLDNN conv + elementwise_add fusion: output and elemwise param share data in conv primitive. Output is properly allocated

MKLDNN conv + elementwise_add fusion: inputs in pass modified. Support for new conv parameter. UTs corrected

MKLDNN conv + elementwise_add fusion: added reachability tests, inputs and outputs in graph nodes are transformed

MKLDNN conv + elementwise_add fusion: implementation of patterns refarctored, applied to graph. UTs added

test=develop

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* add MKL-DNN placement pass

This patch also refactors conv+bn (includes changes from PR
https://github.com/PaddlePaddle/Paddle/pull/13926)
updated to use the mkldnn-placement-pass.

test=develop

* remove redundant pass list

* add comment on the default first pass

* fix test for conv+relu mkldnn fuse

test=develop

This adds a `TODO` comment according to https://github.com/PaddlePaddle/Paddle/issues/13550#issuecomment-430133585

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