Add construct_batch and transform_program functions to workflow module

[albi3ro]

See #5058 for a prototype of the end state.

This PR adds two functions qml.workflow.construct_batch and qml.workflow.transform_program.

transform_program(qnode, level) takes a qnode and a "level" and returns a transform program. The level keyword argument will be used through construct_batch, draw, draw_mpl, and specs and indicates a selection from the full transform program.

construct_batch(qnode, level)(*args, **kwargs) takes a qnode and a level and returns a callable with the same signature as the qnode. It then applies the transform program corresponding to level.

Additional minor changes and helpers:

• qml.transforms.core.expand_fn_transform: In order to place qml.Device.expand_fn into the transform program easily, I added a quick function to convert from a tape->tape function to a transform.

• TransformContainer.__repr__: This just makes my life easier when working with transforms.

• Default the is_informative keyword argument to False instead of None when creating a transform. This way a transform created with TransformContainer has the same default value of is_informative. This was causing me headaches when comparing transforms in testing.

• Slicing into a TransformProgram with a slice object returns another TransformProgram instead of a list. Ex prog[0:4]

• Defined __contains__ for TransformProgram. This just made my life easier when playing around and testing things out. example: qml.compile in my_program.

• Added some documentation on defined dunder methods in TransformProgram

[trbromley]

Thanks @albi3ro! This looks great. I think it's still WIP so I'll come back to look at this later.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (f19d976) 99.69% compared to head (b6ab690) 99.68%.
Report is 1 commits behind head on master.

Additional details and impacted files

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- Hits        35801    35598     -203     
- Misses        111      112       +1     

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[lillian542]

Looks great to me - transform_program and construct_batch both look super useful 🚀

[timmysilv]

really excited to have this in 🍾 most of my comments are about style or docs, lmk what you think

[timmysilv]

I'd like some finality on what to do with expand_fn_transform before this merges, but otherwise I think it's good to go. I'm fine to not make a public solution to the single-tape transform in this PR, but if that's the case, I would prefer that we keep this one private until we have a proposed UI for it

[timmysilv]

love it

[trbromley]

Nice! Approved, subject to my comments.

To check I understand, this will be used for the drawing and specs functionality?

[albi3ro]

Nice! Approved, subject to my comments.

To check I understand, this will be used for the drawing and specs functionality?

Yes this will be for drawing and specs.