fix: Refactor Qubit and QubitSet to a separate module

[kshitijc]

Issue #, if available:

Description of changes:

• The existing implementation has circular dependencies between the circuit and pulse modules. Trying to execute the statement from braket.pulse import PulseSequence causes an ImportError. The following dependencies are executed:

from braket.pulse.pulse_sequence import PulseSequence (braket/pulse/__init__.py) -> from braket.circuits.qubit_set import QubitSet (braket/pulse/pulse_sequence.py) -> from braket.circuits import circuit (braket/circuits/__init__.py) -> from braket.pulse import PulseSequence (braket/circuits/circuit.py)

Conceptually, the hierarchy of the abstractions is:

Circuit
  |   \
  |   PulseSequence
  |  /
  | /
  \/
Qubit

However, today the Circuit and Qubit abstractions live in the same module causing circular dependencies:

Circuit & Qubit
   \    /\
   \/   / 
  PulseSequence

This change abstracts out the Qubit classes to its own module to align with the desired heirarchy of abstractions.

Testing done:

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[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (1acf476) 100.00% compared to head (b765b92) 100.00%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #717   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files          123       126    +3     
  Lines         8185      8189    +4     
  Branches      1827      1827           
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+ Hits          8185      8189    +4     

Files	Coverage Δ
src/braket/_sdk/_version.py	100.00% <100.00%> (ø)
src/braket/circuits/ascii_circuit_diagram.py	100.00% <100.00%> (ø)
src/braket/circuits/circuit.py	100.00% <100.00%> (ø)
src/braket/circuits/compiler_directive.py	100.00% <100.00%> (ø)
src/braket/circuits/gate.py	100.00% <100.00%> (ø)
src/braket/circuits/gate_calibrations.py	100.00% <100.00%> (ø)
src/braket/circuits/gates.py	100.00% <100.00%> (ø)
src/braket/circuits/instruction.py	100.00% <100.00%> (ø)
src/braket/circuits/moments.py	100.00% <100.00%> (ø)
src/braket/circuits/noise.py	100.00% <100.00%> (ø)
... and 21 more

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[speller26]

Really happy to see this split out; the only thing that gives me pause is the name of the module. braket.qubits Forces everything in the module to be a qubit register, but what if we introduce qudits down the line? What if we have a module that can store any fundamental unit? Don't know what I'd all it though.

[laurencap]

I like @peterkomar-aws's idea of the directory organization being braket/registers/qubit.py. Either way, can we add a line to the init file to allow users to import as from braket.qubits import Qubit? I find it hard as a user (with the existing documentation) to find the right paths, so lets keep it simple where possible.