Created by: qingqing01

PR types

Function optimization

PR changes

APIs

Describe

1. Unify the metrics implementation between low-level and high-level API.
2. Change function name add_metric_op in Metric to compute.
3. Return scalar, not [scalar] in update() and accumulate() of Accuracy.
4. Refine and add comments for sub-function in Accuracy.
   • Add comments for compute, update, reset..
   • Add standalone usage and usage with Model API for Metrics
5. Add unit testing in python/paddle/tests