ENH: Annotate optimizer attribute type DiffusionGPR

Annotate the `optimizer` attribute type in the DiffusionGPR GPR child
class.

Fixes:
```
src/nifreeze/model/gpr.py:234: error:
 "DiffusionGPR" has no attribute "optimizer"  [attr-defined]
```

raised for example in:
https://github.com/nipreps/nifreeze/actions/runs/12437972140/job/34728973936#step:8:82

src/nifreeze/model/gpr.py

@@ -25,7 +25,7 @@
 from __future__ import annotations
 
 from numbers import Integral, Real
-from typing import Callable, ClassVar, Mapping, Sequence
+from typing import Callable, ClassVar, Mapping, Optional, Sequence, Union
 
 import numpy as np
 from scipy import optimize
@@ -153,6 +153,8 @@ class DiffusionGPR(GaussianProcessRegressor):
 
     """
 
+    optimizer: Optional[Union[StrOptions, Callable, None]] = None
+
     _parameter_constraints: ClassVar[dict] = {
         "kernel": [None, Kernel],
         "alpha": [Interval(Real, 0, None, closed="left"), np.ndarray],