Created by: baojun-nervana

This PR includes the followings:

1. cached compiled ngraph function instead function, this will have some performance benefit to avoid recompile each time
2. Added a basic cache management by clearing the cache when it reaches the capacity cap (set is as 5 for now); this is necessary for BERT model training
3. Simplified some logic and made the code concise; Original it contains a cache off mode for debug purpose, it was replaced by forcing cache clearing.