Simplify qualcomm qnn backend setup

[kimishpatel]

🚀 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

No response

Additional context

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RFC (Optional)

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cc @cccclai @cbilgin

[GregoryComer]

On a related note, we can also integrate the AOT python extensions that QNN builds into our setup.py build logic to avoid the need to manually run those commands and make it work when not running from the executorch directory. I'm hoping to get to that as part of the final pre-GA build cleanup - tracking here: #13778.

Having the SDK managed automatically would be very convenient.

[cccclai]

#13778

We're actually working on it see #13733 it may not include the pybinding just for now