Define a __str__ method for the class QuantumModel

[giovanniminutoNL]

Describe the feature

The feature addresses the challenge of retrieving key attributes of a QNN(QuantumModel) created using either a standard constructor or config. Currently, there is no straightforward method to extract basic details like the number of qubits (n_qubits), the depth of the ansatz, or more advanced properties like the encoding strategy (referred to as BasisSet in Qadence), the operations used in the feature map or in the ansatz.
Introducing a __str__ method that generates a string representation of a QNN containing the most relevant features of the QNN would resolve this limitation and improve usability.

It should be implemented because

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Additional context

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Would you like to work on this issue?

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