Add amplitude damping noise model for density-matrix simulation

[has9sayed]

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Context (if applicable):
Description of the change:

Added amplitude damping as a built-in noise model in Graphix, following the same structure as DepolarisingNoiseModel. It introduces single- and two-qubit Kraus channels, AmplitudeDampingNoise / TwoQubitAmplitudeDampingNoise, and AmplitudeDampingNoiseModel, with exports at the package level.

Related issue:
Closes: #497

[codecov]

Codecov Report

❌ Patch coverage is 90.78947% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.87%. Comparing base (8bf1ebe) to head (5031397).

Files with missing lines	Patch %	Lines
graphix/noise_models/amplitude_damping.py	89.39%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #533      +/-   ##
==========================================
+ Coverage   88.85%   88.87%   +0.01%     
==========================================
  Files          49       50       +1     
  Lines        7135     7210      +75     
==========================================
+ Hits         6340     6408      +68     
- Misses        795      802       +7     

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[pranav97nair]

Thanks for your contribution. As you may have noticed, two other pull requests (#529 and #530) already address the same issue.

Of course, the definition of the noise classes is straightforward, and the more interesting part is writing the test functions to ensure accurate simulation of the new noise model. In this respect, could you clarify how your contribution differs from, or improves upon, the previous works? In particular, please explain why you consider your approach to be better.

You may of course update your submission if you wish!