[cppyy] Set LowLevelView itemsize to sizeof(T) for fixed multidim arrays

[aaronj0]

For fixed contiguous C arrays (e.g. float a[2][2]), the ndim > 1 path in CreateLowLevelViewT was unconditionally setting view.itemsize to sizeof(void*), which is correct for pointer-to-pointer arrays but does not work for fixed arrays. Fixes #21378

[vepadulano]

This is great! Could you also add a test?

[aaronj0]

This is great! Could you also add a test?

neat, this actually revealed a bug in a test pyunittests-bindings-pyroot-pythonizations-pyroot-array-numpy-views that I wrote for fixed-width integer low level views: #18492 :)

Back then, I didn't realise why I had to read the buffer as 1D and reshape the resulting Py_buffer but now it all makes sense...
In the test, we see the same case, view.len = 16, and view.itemsize = 8 so frombuffer saw 2 elements of size 8, but count=22 forced it to reinterpret the raw bytes as 22 elements anyway. That's a bad pattern..

With this patch, frombuffer correctly sees 4 floats, but count=22 now exceeds the buffer size (as it should). Now that the underlying issue is fixed, we can just use np.array(cpp_arr, dtype=dtype[0]) and it works as expected!

[github-actions]

Test Results

    21 files      21 suites   2d 22h 13m 21s ⏱️
 3 807 tests  3 804 ✅ 1 💤 2 ❌
72 874 runs  72 863 ✅ 9 💤 2 ❌

For more details on these failures, see this check.

Results for commit 6d9f885.

♻️ This comment has been updated with latest results.

[vepadulano]

Thank you!