This commit does/fixes the following:
1. Adds unit tests for byte and patch packing to ensure it works correctly
2. Fixes a bug where for batches that end up with <max_length number of bytes (e.g., short patches), the mask was including elements that had value pad_id. This fixes the mask by setting it to be !=pad_id, if its not specified.
3. Correctly handles the last batch, where previously it would crash. This didn't affect training since we had enough data and/or looped iterators, but for evaluation perplexity, it comes up if we validation on an entire file.
4. Correctly forward the mask if it exists for byte packing
Test Plan:
```
pytest bytelatent/
```
Testing these changes more thoroughly in a stacked PR that fixes evals
203bff3696