Move qml.device to qml.devices

[Shiro-Raven]

Context:
The root __init__ file is bloated with device instantiation code through the qml.device function and its helper functions. It is also contaminating the namespace with unrelated third-part packages imports needed for its functionality. The code doesn't need to live in this file since we have a dedicated qml.devices module now.

Description of the Change:
The relevant code is moved to the __init__ of the qml.devices module and reference in the root __init__ file and the docs are adapted. The imports are also pushed inside the relevant functions to avoid contaminating the namespace.

Benefits:
More localized functional scopes.

Possible Drawbacks:
None

[sc-65678]

[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit doc/releases/changelog-dev.md with:

• A one-to-two sentence description of the change. You may include a small working example for new features.
• A link back to this PR.
• Your name (or GitHub username) in the contributors section.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.65%. Comparing base (be355e0) to head (98ba139).
Report is 295 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6030      +/-   ##
==========================================
- Coverage   99.65%   99.65%   -0.01%     
==========================================
  Files         427      428       +1     
  Lines       41150    40865     -285     
==========================================
- Hits        41010    40724     -286     
- Misses        140      141       +1     

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[albi3ro]

:)

[lillian542]

Looks good to me!