Code for "an empirical study of spatial attention mechanisms in deep networks" (#730)

* update configs and related code

* remove debug info

* fix style issues

* fix unnecessary changes

* move generalized_attention.py from ops/dcn to models/plugins

* change the arguments name of generalized attention, add related description and update resnet backbone

* revert the change of weight_init.py

* format touched files by yapf

* change default data_root

* update code for passing flake8 test

* delete unnecessary __init__.py

* Change the argument order of resnext.__init__()

* remove constant_init

* simplifying the init of generalized_attention module

* remove unused import

* format the code

* change the name of config folder, add more configs, add README.md

* rename num_head to num_heads

* minor formatting