[Refactor] refactored eager_gen.py PR #1

[jim19930609]

PR types

Function optimization

PR changes

Others

Describe

Refactored automatic code generation for final state eager dygraph functions. In additional to code polish, three major changes are further involved:

1. Added codegen_utils.py which includes common helper functions, and base class of YamlGenerator and FunctionGenerator.
2. Grouped up logics for Dygraph Function Generation & GradNode Generation and uses a class named "DygraphSingleFunctionGenerator" to manage all the corresponding functions.
3. Code generation across different APIs (normal api, sparse api, string api, etc) were managed by DygraphYamlGenerator

[XiaoguangHu01]

LGTM