Refactor Identity op to be part of qubit and cv modules

[dime10]

Context: The Identity operation currently is added directly to the ops module, not being part of either ops.qubit or ops.cv, as it is common to both. However, this means there is no easy way to query which ops are supported by PennyLane on qubits only, or to selectively import them.

For example, the list given by qml.ops.qubit.ops does not include the string "Identity", and from qml.ops.qubit import * does not make the class Identity available.

Description of the Change:

• Move the Identity class to a separate file (from ops/__init__.py).
• Make the Identity operation part of both the qubit and cv modules.

Benefits: All supported qubit operations are now part of the qubit module, and similarly for the supported continuous variable operations.

Possible Drawbacks:

Related GitHub Issues:

[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.

[dime10]

[sc-11906]

[codecov]

Codecov Report

Merging #1956 (52039d5) into master (a14abd1) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1956   +/-   ##
=======================================
  Coverage   98.81%   98.81%           
=======================================
  Files         224      225    +1     
  Lines       17307    17310    +3     
=======================================
+ Hits        17102    17105    +3     
  Misses        205      205           

Impacted Files	Coverage Δ
pennylane/ops/__init__.py	100.00% <100.00%> (ø)
pennylane/ops/cv.py	100.00% <100.00%> (ø)
pennylane/ops/identity.py	100.00% <100.00%> (ø)
pennylane/ops/qubit/__init__.py	100.00% <100.00%> (ø)

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[antalszava]

Looks good to me! 🎉 Thank you for catching this and tidying it. 🙂