Flexible selector implementation

**Is your feature request related to a problem? Please describe.**
Forte provides the selector interface that can take data packs from multi packs. However, there can be many other use cases not covered by this simple approach, such as selecting a subset of data packs (as multi packs) from the multi-packs. Supporting such selection can simplify the processor implementation and allow one to change the target of operation at any time.

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**Additional context**
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