[BUG] the saved model only have parameters of classification head

### Describe the bug

training with task='regression',but the save model only has the parametrs of 'classification_head.out_proj.weight'

### Uni-Mol Version

unimolv1 and v2

### Expected behavior

the saved model should hace 'regression_head'

### To Reproduce

_No response_

### Environment

pyhton 3.10

### Additional Context

_No response_

Start by reproducing training with task='regression' on Uni-Mol v1 or v2 using Python 3.10, then inspect the saved model parameters. Confirm whether the checkpoint contains only classification_head.out_proj.weight; done means a regression_head is saved for regression tasks.

Written by the indexing model from the issue text.