Documentation compatibility with qutip-v5

[BoxiLi]

• Remove some unnecessary branching that was used for the compatibility of qutip-v5. They are no longer needed in the new alpha version.
• Update the pulse paper examples to be compatible with qutip-v5.
• Update the documentation to be compatible with qutip-v5.

Note that doctest still doesn't fully pass with qutip-v5 because of the spaces in printed Qobj._repr_. But this should be fine if we only build the doc against qutip-v4.7 for now.

[BoxiLi]

@hodgestar
DO you think if worth have something like this in qutip-v5?

qutip-qip/src/qutip_qip/utilities.py

Lines 8 to 16 in 8c3af66

	def _qobj_compatibility_helper(qobj):
	"""
	Make qobj generated by operations in qutip-v5 identical to the one by qutip-v4.
	Here, if `qobj` is a number (e.g. generated by a bra-ket multiplication), it is replaced by a Qobj.
	"""
	if parse_version(qutip.__version__) >= parse_version("5.dev"):
	if np.isscalar(qobj):
	return qutip.Qobj([[qobj]], dims=[[1], [1]])
	return qobj

[hodgestar]

We should definitely have a nice solution for cases like q.dag() * q which return a scalar in v5 and a Qobj in v4, but I don't think that solution can be in qutip. We could add a function to v5, but no such function is in v4 and we're not planning a 4.8. So the function probably has to be in libraries that use qutip, like qutip_qip.

Rather than the function you have now, I recommend creating a qutip_qip.compat module (so one can easily remove these later) and inside having a function like:

def to_scalar(qobj_or_scalar):
    if isinstance(qobj_or_scalar, Qobj):
        if qobj_or_scalar.dims == [[1], [1]]:   # <-- maybe this check should also handle more dims?
            return qobj_or_scalar[0, 0]
        else:
            raise ValueError(...)
    return qobj_or_scalar

This avoids explicit version checks and implements a more specific set of functionality.

[BoxiLi]

Yes, the example you provide is better, avoiding the version check. There are indeed other cases instead of just dims=[[1],[1]] but this covers >99% of situations.

And now thinking about it again. You are also right that unless we can have this for both qutip-v4 and v5, it is not gonna be sufficient. Maybe we could just provide the example above as part of the upgrade guide.