Making sampler composable

[kazewong]

The current workflow of switching between global and local sampler is fixed. This limits the flexibility of dynamic scheduling and early stopping.

It would be useful to wrap both training and sampling into a "strategy block", so the user can composable different training and sampling strategy.

This requires the following work:

• [ ] Defining a strategy class
• [ ] Wrap existing sampler into strategy class
• [ ] Migrate the core of sampler into using strategy class
• [ ] Set default in the current sampler