Fix: Potential Data Leakage in Quantum Data Tutorial.

[OkuyanBoga]

A solution to potential data leakage in #828.

Instead of concatenating train and test sets, they should be separately dealt with when getting a stilted dataset:

In lines L745-752:

y_train_new = get_stilted_dataset(S_pqk, V_pqk, S_original, V_original)
y_test_new = get_stilted_dataset(S_pqk_test, V_pqk_test, S_test_original, V_test_original)

where spectrum is calculated separately for test set:

S_pqk_test, V_pqk_test = get_spectrum(
    tf.reshape(x_test_pqk, [-1, len(qubits) * 3]))

S_test_original, V_test_original = get_spectrum(
    tf.cast(x_test, tf.float32), gamma=0.005)

print('Eigenvectors of pqk kernel matrix for test:', V_pqk_test)
print('Eigenvectors of original kernel matrix for test:', V_test_original)

Closes #828.

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[mhucka]

@OkuyanBoga Thank you for this contribution. Would you be able to resolve the (simple) conflicts that have arisen? I will then review the PR.