[JIT] Fix torch.jit.script for functions with many decorators (#87804)
Summary:
Python's function parsing from the `ast` module records the line number of the function definition, not the first decorator. So this diff fixes crashes like this:
```
IndexError: vector::_M_range_check: __n (which is 10) >= this->size() (which is 8)
```
Test Plan: New unit test
Differential Revision: D40726352
Pull Request resolved: https://github.com/pytorch/pytorch/pull/87804
Approved by: https://github.com/tugsbayasgalan, https://github.com/davidberard98
