Code of "GCNet: Non-local Networks Meet Squeeze-Excitation Networks and Beyond" (#598)

* add pytorch 1.1.0 SyncBN support

* change BatchNorm2d to _BatchNorm and call freeze after train

* add freeze back to init function

* fixed indentation typo in adding freeze

* convert SyncBN to BN when eval is called

* add gcb and configs

* fixed line too long

* fixed space and newline

* ignore too long line in config files

* two space before inline comment

* refactor var name

* fixed line too long

* changed SyncBN to backbone only and add non-Sync BN configs

* reformat config to adapt master branch

* reformat change method->type

* remoe some configs

* clean up readme.md

* add benchmark

* fix typo in README

* change sbn to syncbn

* change work dir

* refactor gcb

* fixed line too long

* Code formatting