[`docs`] Improve visualization for vertical parallelism (#28583)

The documentation says "We refer to this Model parallelism as “Vertical” because of how models are typically visualized.", but then visualizes the model horizontally. This change visualizes the model indeed vertically.

docs/source/en/perf_train_gpu_many.md

@@ -285,10 +285,19 @@ following diagram shows an 8-layer model split vertically into two slices, placi
 GPU0 and 4-7 to GPU1:
 
 ```
-===================  ===================
-|  0 | 1 | 2 | 3  |  |  4 | 5 | 6 | 7  |
-===================  ===================
-        GPU0                 GPU1
+================
+| Layer |      |
+|   0   |      |
+|   1   | GPU0 |
+|   2   |      |
+|   3   |      |
+================
+| Layer |      |
+|   4   |      |
+|   5   | GPU1 |
+|   6   |      |
+|   7   |      |
+================
 ```
 
 In this example, when data moves from layer 0 to 3, it's no different from regular forward pass. However, passing data