add note to cost func

[miamico]

Adding a note to the cost_func_scaled_cr function in the cost_funcs module to help clarify the best usage of the fiunction. In particular, if matching layouts are searched for a transpiled circuit with less active qubits than the number of available qubits in the backend, layouts containing all possible permutations of unused qubits will be included. User must be careful that an exponential number of layouts is not retrieved when directly using transpiled circuits instead of deflated ones.

[coveralls]

Pull Request Test Coverage Report for Build 3348830215

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 88.75%

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Totals
Change from base Build 3338525469:	0.0%
Covered Lines:	2414
Relevant Lines:	2720

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💛 - Coveralls

[kevinsung]

I believe the block under "Note:" needs to be indented one level.

[nbronn]

Actually since the point of this cost function is to evaluate the final circuit that has gone through all the transpilation passes (and includes pulse-level data about the selected qubits that would be discard by deflateion), perhaps it makes sense to only run this cost function on circuits where circ.num_qubits == backend.num_qubits is True.

[miamico]

I think the problem is not just with the fact that there can be redundant layouts if some qubits are not used (but should be included in the transpired circuit). The cost itself is not evaluated correctly as the loop over layouts doesn't have any effects. The cost of the circuit will be the one of the given transpiled circuit (transpiled to a particular layout) regardless of the layout in the iteration. So we shouldn't have any layout input here and assume the circuit has already been fitted to work for a particular layout for which we calculate the cost. If we want to go evaluate different layouts with the current cost function, we should re-transpile the circuit to each layout

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