ModuleNotFoundError: No module named 'torchtitan.components.gpa'

No module named `torchtitan.components.gpa` when importing `torchtitan`.

```bash
(muloco) ubuntu-420GP-TNAR:~/llm/MuLoCo$ python -c "import torchtitan, torchft; print('imports ok')"
Traceback (most recent call last):
File "", line 1, in
File "/home/lishuai/llm/MuLoCo/torchtitan/torchtitan/__init__.py", line 13, in
import torchtitan.models # noqa: F401
^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/lishuai/llm/MuLoCo/torchtitan/torchtitan/models/__init__.py", line 10, in
import torchtitan.models.llama3 # noqa: F401
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/lishuai/llm/MuLoCo/torchtitan/torchtitan/models/llama3/__init__.py", line 10, in
from torchtitan.components.lr_scheduler import build_lr_schedulers
File "/home/lishuai/llm/MuLoCo/torchtitan/torchtitan/components/lr_scheduler.py", line 15, in
from torchtitan.components.optimizer import OptimizersContainer
File "/home/lishuai/llm/MuLoCo/torchtitan/torchtitan/components/optimizer.py", line 23, in
from torchtitan.components.gpa import GPA
ModuleNotFoundError: No module named 'torchtitan.components.gpa'
```