ggml-qnn: refine ggml backend subsystem

[zhouwg]

This PR is equal to a PR in upstream GGML communtiy:

ggerganov/llama.cpp#7641

Unfortunately, the PR in upstream was closed by the maintainer of ggml backend subsystem very quickly/immediately less then 1 minute after I summited this PR in upstream llama.cpp.

I totally disagree with what the maintainer of ggml backend subsystem said(because some special backends only need system memory):

"There are too many things wrong here to list. At the most basic level, this approach will not work because backends typically have a memory that is not accessible from other backends, and when switching to a different backend it is necessary to ensure that all the tensors required to evaluate the graph are available in the backend memory. This is the main job of ggml_backend_sched."

[zhouwg]

whisper and llm and minicpm-v infereence works fine with this PR(mixed inference between Qualcomm's CPU&GPU / CPU&NPU).

next step:
(1) buf fix in JNI layer, this is not important in development stage(there are three known bugs in JNI layer);

(2)QNN performance fine-tuning: focus on matmul because the performance of QNN's matmul is 2x - 10x then original GGML's matmul, all the other GGML OPs will be computed by the original GGML in the CPU side, just offload the matmul in QNN side.