enable pipeline checkpoint loading mode (#3629)
In cpu ram limited machine, loading checkpoint at the start up may
cause oom as all rank in the same node are loading the opt state
in the same time. So for this scenario, we make a choice that loading
checkpoint could be made pipeline way.
Signed-off-by: Lei Wen <wenlei03@qiyi.com>
Co-authored-by: Lei Wen <wenlei03@qiyi.com>
