Add a combine_global_phases transform

[mudit2812]

Important Note

⚠️ This issue is part of an internal assignment and not meant for external contributors.

Context

We would like to add a transform to PennyLane that combines all qml.GlobalPhase gates in a circuit into a single one applied at the end. This can be useful for circuits that include a lot of qml.GlobalPhase gates, which can be introduced directly during circuit creation, decompositions that include qml.GlobalPhase gates, etc.

Implementation Details

This task is to add a new transform called combine_global_phases. This transform should be added to the pennylane/transforms folder in a new file called combine_global_phases.py.

This transform should:

• Find all instances of qml.GlobalPhase in a QuantumScript, regardless of the wires that it is applied to
• Sum the phase for each qml.GlobalPhase gate
• Create a new QuantumScript that includes all the operations and measurements other than qml.GlobalPhase of the original QuantumScript
• Apply a qml.GlobalPhase as the last operation in the new QuantumScript with the phase being equal to the sum computed earlier. Wires do not need to be specified for this qml.GlobalPhase

Relevant functions include:

• qml.GlobalPhase
• qml.transforms.core.transform
• Any transforms from the pennylane/transforms folder can be used as reference

Additional Requirements

A completed PR would:

• Include a docstring for the new combine_global_phases transform with details about what the transform does, arguments, return values, and a code example that showcases the transform
• Include a changelog entry at doc/releases/changelog-dev.md. Make sure to include your name in the list of contributors at the bottom!
• Add tests to a new file tests/transforms/test_combine_global_phases.py. The tests should include an adequate suite of both unit and integration tests.
• Pass all existing tests including black and pylint checks