Fix loading weights in n-dim dense -> 1x1 conv

[vloncar]

Description

In a change from using the readder to storing weights as attributes we missed the case of creating PointwiseConv nodes from n-dimensional Dense layer. This fixes it and adds a test for the future.

Type of change

• [x] Bug fix (non-breaking change that fixes an issue)

Tests

Added a test in test_multi_dense.py

Checklist

• [x] I have read the guidelines for contributing.
• [x] I have commented my code, particularly in hard-to-understand areas.
• [x] I have made corresponding changes to the documentation.
• [x] My changes generate no new warnings.
• [x] I have installed and run pre-commit on the files I edited or added.
• [x] I have added tests that prove my fix is effective or that my feature works.