Represent non-fixed params as NaN for QkTargetOp

[raynelfss]

Summary

Fixes #15438
Due to our decision of provisionally exposing parameters in QkTargetOp as double[] we mistakenly filtered out any non-fixed parameters instead of showing a placeholder value. This PR provisionally exposes any non-fixed parameter instance as NaN until we work out a much cleaner way of exposing the parameters to C. This PR now exposes QkParam directly by having the params attribute be an array of QkParam.

Details and comments

• Add release note.

[coveralls]

Pull Request Test Coverage Report for Build 20375166055

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Details

• 3 of 7 (42.86%) changed or added relevant lines in 1 file are covered.
• 37 unchanged lines in 5 files lost coverage.
• Overall coverage increased (+0.003%) to 88.3%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
crates/cext/src/transpiler/target.rs	3	7	42.86%

Files with Coverage Reduction	New Missed Lines	%
crates/cext/src/transpiler/target.rs	1	86.59%
crates/qasm2/src/lex.rs	5	91.77%
crates/circuit/src/parameter/symbol_expr.rs	9	72.94%
qiskit/providers/basic_provider/basic_simulator.py	10	96.77%
crates/qasm2/src/parse.rs	12	96.62%

Totals
Change from base Build 20304825545:	0.003%
Covered Lines:	96705
Relevant Lines:	109519

---

💛 - Coveralls

[Cryoris]

From a bugfix POV I don't think we need to backport this (since we can't create a C Target object that would have params of unknown length). But from an interface POV this PR changes an all-free params from a NULL pointer to an array of f64::NAN and it would be good to not change this interface between 2.3 and 2.4, which makes it backportable IMO.

[Cryoris]

This last part isn't true anymore, is it? If all parameters are free then we'd still have an array with all f64::NAN entries, right?

[raynelfss]

I guess I could've worded it better. But what I meant was if the operation doesn't accept any parameters.

[qiskit-bot]

One or more of the following people are relevant to this code:

• @Qiskit/terra-core