Simplify qualcomm qnn backend setup

🚀 The feature, motivation and pitch

As we try to move to a place where QNN SDK does not need to be downloaded, explicitly, from user, we want the setup to be simplified as well. At the moment we need to do two things. 1 setup QNN_SDK_ROOT and 2 change LD_LIBRARY_PATH https://github.com/pytorch/executorch/blob/main/docs/source/backends-qualcomm.md#setup-environment-variables. Neither of the two should be needed. Specifically the second part. If QNN_SDK_ROOT is already available why do we not use that to set this path. Specifically change https://github.com/pytorch/executorch/blob/main/backends/qualcomm/utils/utils.py#L1036 to query the QNN_SDK_ROOT to set the path. SImilar to here https://github.com/pytorch/executorch/blob/main/backends/qualcomm/utils/utils.py#L1176.

However, more importantly, I would like users to not even have to setup QNN_SDK_ROOT. When we get to a point where SDK is downloaded automatically, we should set this path ourselves.

cc: @cccclai @shewu-quic

Alternatives

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Additional context

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RFC (Optional)

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cc @cccclai @cbilgin

Start in backends/qualcomm/utils/utils.py at the referenced locations around lines 1036 and 1176, then read the setup instructions in docs/source/backends-qualcomm.md. Determine how the current environment variables are consumed and define completion for both deriving the library path from QNN_SDK_ROOT and the longer-term automatic SDK setup, without leaving the setup behavior ambiguous.