Alternative solution to representation caching

[apchytr]

Opening for discussion

Context: This PR is meant as a proposed alternative to representation caching. By using 'lazy evaluation' in representation/ansatz we can avoid having potentially costly computations just to initialize the representation for calls such as obj.representation.__class__.

Description of the Change: Ansatz no longer calls backend methods in the init. Instead, the original parameters are stored (mat, vec, array for PolyExpAnsatz or array for ArrayAnsatz) and backend methods such as astensor and at_least3d are only called when the property itself is called (then subsequently stored to avoid repeat calls). In addition to this the representation/ansatz can now also accept a function and some kwargs in substitution of specifying an (A, b, c) through a from_generator class method.

Benefits: Based on some initial profiling initializing a representation/ansatz is more efficient.

[codecov]

Codecov Report

Attention: Patch coverage is 97.77778% with 2 lines in your changes missing coverage. Please review.

Project coverage is 87.87%. Comparing base (31a1f8a) to head (ad8848d).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #431      +/-   ##
===========================================
+ Coverage    87.80%   87.87%   +0.06%     
===========================================
  Files           81       81              
  Lines         6230     6282      +52     
===========================================
+ Hits          5470     5520      +50     
- Misses         760      762       +2     

Files	Coverage Δ
mrmustard/lab_dev/circuit_components_utils.py	100.00% <100.00%> (ø)
mrmustard/lab_dev/states/base.py	96.91% <100.00%> (ø)
mrmustard/lab_dev/states/states.py	100.00% <100.00%> (ø)
...mustard/lab_dev/transformations/transformations.py	100.00% <100.00%> (ø)
mrmustard/physics/representations.py	97.64% <100.00%> (ø)
mrmustard/physics/triples.py	100.00% <100.00%> (ø)
mrmustard/physics/ansatze.py	95.31% <97.14%> (+0.23%)	⬆️

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[ziofil]

Beautiful concept, I really like it! Before we lock it in could we keep A,b,c in the init of Bargmann, and have a separate class method that takes fn, **kwargs instead? (it's a bit odd to have A,b,c and (e.g.) cov, means as arguments of the init)