[XLA:TPU] HostOffloader: Refactor HandleInputStreaming to occur before tracing the graph for custom calls.
[XLA:TPU] HostOffloader: Refactor HandleInputStreaming to occur before tracing the graph for custom calls.

Currently, input streaming gets handled in arbitrary order, after we have already possibly searched the graph for the corresponding streamed custom calls. This refactors the input streaming to occur before any graph searching, which makes more sense and prepares for output streaming.
