Error in dense-gemm.ipynb in Tiled Matrix Multiply section

Two issues:

First, the logging statement
```
logger(f"Processing Z({m},{n}) = {z[m][n].payload}")
```
seems misplaced, the variable `n` is not defined yet at that point.


Second, tensor Z is not being filled with zeroes. The "hack" to fill in entries of Z does not seem to work anymore.