add param_attr parameter for img_conv_groups(...) in /python/trainer_config_helpers/networks.py

update gradient operator registry mechanism

Implement distributed training save model, improve master.NewClient i…

do not do log.Errorln when checkpoint is not found (which is normal)

OLD: op_type -> grad_op_creator

NEW: grad_op_type -> grad_op_creator
     op_type -> grad_op_type

Backward

Fix doc

Replace `OperatorPtr` with `std::shared_ptr<OperatorBase>`

Remove ScopePtr and OperatorPtr

* ScopePtr means pointer of scope, but it can be shared or uniqued.
Change it to std::shared_ptr<Scope> to make code better to read.