TMA selection of get_best_configs in gemm.py

```
for i in ('A', 'B') if best_block_m > best_block_n else ('B', 'A'):
if m >= 512 and is_multicast_legal[i]:
best_tma_multicast_config = (2, i == 'A')
break
```
when best_block_m <= best_block_n, I think need to use n instead of m to choose. Need to change to below:

```
for i in ('A', 'B') if best_block_m > best_block_n else ('B', 'A'):
dim_size = m if best_block_m > best_block_n else n
if dim_size >= 512 and is_multicast_legal[i]:
best_tma_multicast_config = (2, i == 'A')
break

```

Open gemm.py and locate get_best_configs and the shown TMA multicast-selection block. Trace how best_block_m, best_block_n, m, and n are used, then run the repository’s relevant GEMM or configuration tests if available. Done means the dimension checked for the 512 threshold matches the selected block direction when best_block_m is not greater than best_block_n.

Written by the indexing model from the issue text.