[core][experimental] Avoid serialization for data passed between two tasks on the same actor

[Stephanie Wang]

Description

#44259 supports multiple tasks on the same actor in one DAG. However, if the same actor passes data from one task to the next, it currently still requires a shared memory channel, which means we need to copy and serialize/deserialize. We should add a LocalChannel type that just buffers the Python value to avoid this.

Use case

No response