BUG: The coordinate ordering error in the Polygen implementation

Hi @saran-t,

**File**: [polygen](https://github.com/deepmind/deepmind-research/tree/master/polygen)/data_utils.py
**Bug:** Reordering twice, so that the order is wrong. (Different from the idea in the paper)

In data preparation part, function(Load_process_mesh(), line 289) is called to read and process data files(.obj). The coordinate has already been reordered. **(xyz -> zxy)**
```
# Transpose so that z-axis is vertical.
vertices = vertices[:, [2, 0, 1]]
```

However, before the training, the coordinates are reordered again **(zxy->yxz)** in function(make_vertex_mode_dataset(), line 45).
```
# Re-order vertex coordinates as (z, y, x).
vertices_permuted = tf.stack([vertices[:, 2], vertices[:, 1], vertices[:, 0]], axis=-1)
```

So the final order of the flattened sequence is **yxz** instead of **zyx** proposed in the paper. (From lowest to highest by z-coordinate)