[dattri.benchmark] Add ResNet-9 benchmark on CIFAR-2

[tingwl0122]

Description

As titled.

1. Motivation and Context

Organize the resnet9/cifar-2 related files to follow the current structure.

2. Summary of the change

• add a test file
• add cifar2-resnet9 benchmark files (model def, train/eval codes)

3. What tests have been added/updated for the change?

• [x] Unit test: Typically, this should be included if you implemented a new function/fixed a bug.