why MyCollator left pad and right pad the features?

From what I understand `__call__` function will return `---xxxxxxxx---` where `x` is word token and `-` is pad token.

The `__call__` function documentation

```
"""
Pad the batch like this to maximize the reuse of kv cache.
E.g.,

xxxxxxxxxxxxxxx--
-----xxxxxxxxxxxxx-------
---xxxxxxxxxxxxxx

("x" is word token, "-" is pad token)
"""
```

https://github.com/facebookresearch/coconut/blob/d1c10b134ad2bded92b0eca4c0a422c513a12dea/dataset.py#L79-L185

Start in dataset.py around the documented __call__ function at lines 79–185 and compare the implementation with the padding example in the issue. Determine whether the behavior is intentional and whether the documentation needs clarification; done means the discrepancy has a clear explanation or the affected documentation is corrected.

Written by the indexing model from the issue text.