GitLab

* Add gradient cumulative optimizer

fixes #190

* Update optimizer.py

* Update optimizer.py

* fix loss scale improperly in last equivalent_iter

* Add `GradientCumulativeOptimizerHook` in `__init__.py`.

* Add docstring of `GradientCumulativeOptimizerHook`.

* Add type check, BN warning and resume warning. And fix typo, lint the
code.

* Add unit test

* Update docstring example.

* Change GradientCumulativeOptimizerHook `__init__` arguments.

* Add GradientCumulativeOptimzierHook unit tests with IterBasedRunner.

* Add GradientCumulativeFp16OptimizerHook.

* Add unit tests of GradientCumulativeFp16OptimizerHook

* Use '!=' instead of '>' to determine resume
Co-authored-by: Zhiyuan Chen <this@zyc.ai>