fix step in adam (#1823)
* fix step in adam
* fix backward compatibility and add unittest
* add unittest
* fix unbounded error when there are more than 1 param groups
* fix typo
* remove trailing whitespace
* fix end of file
Co-authored-by: Shuai Zheng <shzheng@amazon.com>
Co-authored-by: Olatunji Ruwase <olruwase@microsoft.com>
