Remove warnings generated in test_graphmask_explainer.py and test_laplacian_lambda_max.py

[drivanov]

With this fix, the following warning:

test/explain/algorithm/test_graphmask_explainer.py: 12960 warnings
  /usr/local/lib/python3.10/dist-packages/torch_geometric/explain/algorithm/graphmask_explainer.py:165: DeprecationWarning: `np.math` is a deprecated alias for the standard library `math` module (Deprecated Numpy 1.25). Replace usages of `np.math` with `math`
    beta * np.math.log(-gamma / zeta))


test/transforms/test_laplacian_lambda_max.py::test_laplacian_lambda_max
test/transforms/test_laplacian_lambda_max.py::test_laplacian_lambda_max
test/transforms/test_laplacian_lambda_max.py::test_laplacian_lambda_max
test/transforms/test_laplacian_lambda_max.py::test_laplacian_lambda_max
  /usr/local/lib/python3.10/dist-packages/torch_geometric/transforms/laplacian_lambda_max.py:65: DeprecationWarning: Conversion of an array with ndim > 0 to a scalar is deprecated, and will error in future. Ensure you extract a single element from your array before performing this operation. (Deprecated NumPy 1.25.)
    data.lambda_max = float(lambda_max.real)

will no longer be generated.