Refactor Interpreter & Subgraph. Preparing to implement control flow ops.

* Expose a few Subgraph-related functions in Interpreter.
  E.g. AddSubgraphs, subgraphs_size, subgraph, primary_subgraph.
* Moved the logic which checks if dynamic tensors exist from
  ModifyGraphWithDelegate to PrepareOpsStartingAt.
  * This is a no-op refactoring with better code structure.
  * Exposing `Subgraph::HasDynamicTensors()` which will be required
    for control flow ops.

PiperOrigin-RevId: 223464887