[Dynamo] throw better error message if assert with non-string message (#97297)
Error message before this PR:
```
torch._dynamo.exc.Unsupported: missing: LOAD_ASSERTION_ERROR
```
After:
```
torch._dynamo.exc.Unsupported: assert with non-string message
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/97297
Approved by: https://github.com/tugsbayasgalan