[Feature] Add helper functions to readily create QNNs

[smitchaudhary]

Currently, constructing QNNs requires a bunch of boilerplate code that needs to be repeated, especially when benchmarking different techniques relating to the definition of feature maps, ansatz and the observable used.

Have helper function build_qnn that creates a Qadence QNN and takes the configuration of the feature map (FeatureMapConfig), of the ansatz (AnsatzConfig) and the observable (ObservableConfig).

fm_config = FeatureMapConfig(
    num_features=1,
    basis_set=BasisSet.CHEBYSHEV,
    reupload_scaling=ReuploadScaling.TOWER,
    feature_range=(-1.2, 1.2),
    feature_map_strategy="digital",
    multivariate_strategy="series",
)

ansatz_config = AnsatzConfig(
    num_layers=2,
    ansatz_type="hea",
    ansatz_strategy="rydberg",
)

obs_config = ObservableConfig(detuning_strength="z")

f = build_qnn_model(
    register=3,
    fm_config=fm_config,
    ansatz_config=ansatz_config,
    observable_config=obs_config,
)

[dominikandreasseitz]

@smitchaudhary, for featuremaps we have: https://github.com/pasqal-io/qadence/blob/main/qadence/constructors/feature_maps.py#L133, for hamiltonians/observables: https://github.com/pasqal-io/qadence/blob/main/qadence/constructors/hamiltonians.py#L52 and for ansatz-related things: https://github.com/pasqal-io/qadence/blob/main/qadence/constructors/ansatze.py#L17. we of course welcome enhancements to those but maybe you have a look at it and communicate with @jpmoutinho about it