1) users configure sharding strategies for operators;
2) framework will propagate the strategies from configured-ops to
non-configured ops using BFS;
3) the propagation goal is to minimize redistribution communication
cost;

1. Add some const variables to represent the input length of operators,
and use them to eliminate some magic numbers;
2. Rename some local variables to distingush other variables in
different ranges;
3. Add some blank rows in python files.

From: @liangzhibo
Reviewed-by: @ginfung
Signed-off-by:

From: @jinyaohui
Reviewed-by: 
Signed-off-by:

From: @ch-l
Reviewed-by: @kisnwang,@stsuteng
Signed-off-by: @stsuteng

From: @huangxinjing
Reviewed-by: @lichen666,@yangzhenzhang
Signed-off-by:

Merge pull request !7608 from Chong/CheckParserInfo