Hexagon Op queue & dispatch optimizations #16820
                
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Optimize dspqueue (used for sending Op requests to the NPU) by doing all response processing in-place.
This removes the need for the dedicated read threads (started internally by the dspqueue library) and essentially
eliminates all polling in that path.
(For the curious, the dspqueue CPU side sources can be found here
https://github.com/qualcomm/fastrpc/tree/main/src/dspqueue)
We can also bump the CPU backend thread counts now for the default use-cases since we still rely on the
CPU for Flash Attention and a few other Ops.