* Inception fusion operator.
* Support horizontal layer fusion in conv_fusion_op.
* Search conv algo strategy for variable-length input.
   search N times and cache the searched algos. For other input, choose the algo of input whose area is closest to this input.

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This reverts commit 7f6d8ace.

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Revert "Merge pull request #14798 from PaddlePaddle/revert-14786-revert-14782-revert-14398-imperative"

This reverts commit b1d3a1c8, reversing
changes made to f1fb64b1.

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* fix api format and examples

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* Update executor.py

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* Update nn.py

* Update nn.py

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* Update nn.py

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*  fix mkdir conflict

*  fix load/save lookup tables

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* add lookup_table_utils

* fix load optimize vars on pserver

* delete lookup table utils

* fix save and load lookup tables

* fix load optimizer var

* fix load optimizer var, test=develop

* fix python 3 style, test=develop

* move lookup_table_utils to contrib utils

1. Non utf-8 characters within comments of OPs may lead to protobuf fail to parse_from_string
2. comment out some ops which not supported on windows
3. cuda libs may not be correctly linked to target on windows

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This reverts commit 21bb9e91, reversing
changes made to 3fa68dc1.

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* hide operator API

* update

* update api.spec

* fix merge

* fix test