Add primitive caching for Pooling forward computation

[TaoLv]

Description

1. Add primitive caching for Pooling forward computation;
2. Enable Avg Pooling support
3. Fix coding style
4. Convergence is validated with cifar10 + ResNet50 (which has avg pooling layer).

Checklist

Essentials

• Passed code style checking (make lint)
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage
• For user-facing API changes, API doc string has been updated. For new C++ functions in header files, their functionalities and arguments are well-documented.
• To my best knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change

Changes

• Feature1, tests, (and when applicable, API doc)
• Feature2, tests, (and when applicable, API doc)

Comments

• If this change is a backward incompatible change, why must this change be made.
• Interesting edge cases to note here