Hi everyone, I am trying to understand the documentation of qml.kernels.

I am currently trying to understand what exactly closest_psd_matrix function do what is its exact use? For example, I tried creating a small example reading your documentation:

```
import numpy as np
K = np.array([[7, 3],[3, -1]])
eigen_values = np.linalg.eigvalsh(K)
import pennylane as qml
K_psd = qml.kernels.closest_psd_matrix(K) # not sure what exactly this line doing?
Result:
tensor([[7.2, 2.4],
[2.4, 0.8]], requires_grad=True)
```

```
K_psd = qml.kernels.closest_psd_matrix(K, fix_diagonal=True)
K_psd
```

Also, when I tried running above lines from the documentation, I am getting the error as below:

```
---------------------------------------------------------------------------
ModuleNotFoundError Traceback (most recent call last)
File ~/anaconda3/envs/pennylane_study/lib/python3.11/site-packages/pennylane/kernels/postprocessing.py:221, in closest_psd_matrix(K, fix_diagonal, solver, **kwargs)
220 try:
--> 221 import cvxpy as cp # pylint: disable=import-outside-toplevel
223 if solver is None:
ModuleNotFoundError: No module named 'cvxpy'
The above exception was the direct cause of the following exception:
ImportError Traceback (most recent call last)
Cell In[14], line 2
1 # However, for quantum kernel matrices, we may want to restore the value 1 on the diagonal
----> 2 K_psd = qml.kernels.closest_psd_matrix(K, fix_diagonal=True)
3 K_psd
File ~/anaconda3/envs/pennylane_study/lib/python3.11/site-packages/pennylane/kernels/postprocessing.py:226, in closest_psd_matrix(K, fix_diagonal, solver, **kwargs)
224 solver = cp.CVXOPT
225 except ImportError as e:
--> 226 raise ImportError("CVXPY is required for this post-processing method.") from e
228 X = cp.Variable(K.shape, PSD=True)
229 constraint = [cp.diag(X) == 1.0] if fix_diagonal else []
ImportError: CVXPY is required for this post-processing method.
```

Thank you.