Fix non convergence in example of usage

[Mboky]

Hi,

The current example of usage does not converge in the amount of iterations specified

import netket as nk
import netket_fidelity as nkf

# Create the Hilbert space and the variational states |ψ⟩ and |ϕ⟩
hi = nk.hilbert.Spin(0.5, 4)
sampler = nk.sampler.MetropolisLocal(hilbert=hi, n_chains_per_rank=16)
model = nk.models.RBM(alpha=1, param_dtype=complex, use_visible_bias=False)
phi = nk.vqs.MCState(sampler=sampler, model=model, n_samples=100)
psi = nk.vqs.MCState(sampler=sampler, model=model, n_samples=100)

# Transformation U
U = nkf.operator.Hadamard(hi, 0)

# Create the driver
optimizer = nk.optimizer.Sgd(learning_rate=0.01)
te =  nkf.driver.InfidelityOptimizer(phi, optimizer, U=U, U_dagger=U, variational_state=psi, is_unitary=True, cv_coeff=-1/2)

# Run the driver
te.run(n_iter=100)

This keeps yielding an infidelity around 0.5. Either adding more iterations or changing to the Adam optimizer fixes this.
With Adam and the same number of iterations, infidelity is consistently smaller than 0.001

[codecov]

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All modified and coverable lines are covered by tests ✅

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+ Coverage   77.13%   77.23%   +0.10%     
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  Files          21       21              
  Lines         678      681       +3     
  Branches      117      117              
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+ Hits          523      526       +3     
  Misses        129      129              
  Partials       26       26              

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[alleSini99]

Thanks!