Make Clifford objects hashable

[itoko]

Summary

Make Clifford and StabilizerTable objects hashable so that they can be a key of mapping objects.

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[coveralls]

Pull Request Test Coverage Report for Build 2887577566

• 4 of 6 (66.67%) changed or added relevant lines in 2 files are covered.
• 3 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.002%) to 84.052%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
qiskit/quantum_info/operators/symplectic/clifford.py	2	3	66.67%
qiskit/quantum_info/operators/symplectic/stabilizer_table.py	2	3	66.67%

Files with Coverage Reduction	New Missed Lines	%
qiskit/quantum_info/operators/symplectic/stabilizer_table.py	1	88.24%
qiskit/extensions/quantum_initializer/squ.py	2	79.78%

Totals
Change from base Build 2882958684:	0.002%
Covered Lines:	56323
Relevant Lines:	67010

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💛 - Coveralls

[jakelishman]

Clifford and StabilizerTable are mutable, so doing this breaks the Python data model. We can't put these methods on these classes in general, but if you need to be able to hash them for some external use where you guarantee you won't mutate them, you can use an extra "lookup function" in your code:

def my_hash(clif):
   return ...

and use the output of this in the set/dict. It's functionally equivalent, but the user has to manually call the hash function when doing stores / lookups. If you need to be able to access the Clifford objects from a key, you can keep a separate table which maps hashes to Clifford objects.

[jakelishman]

(Also, if you need it for speed, #7483 has the fastest implementation of hash I could find as a private method.)

[itoko]

Ah, I missed the most basic thing in the first place, Clifford and StabilizerTable are mutable... Thank you for your comments, @jakelishman As you guessed, I wanted to use lru cache for speeding up Clifford to Instruction conversion. I've understood I need to think of another way to speed up. (I'll also check #7483)