Move type annotation into __init__ (#14943)

Co-authored-by: Carlos Mocholí <[email protected]>

Author: ar90n

src/pytorch_lightning/callbacks/batch_size_finder.py

@@ -31,8 +31,6 @@
 class BatchSizeFinder(Callback):
     SUPPORTED_MODES = ("power", "binsearch")
 
-    optimal_batch_size: Optional[int]
-
     def __init__(
         self,
         mode: str = "power",
@@ -73,7 +71,7 @@ def __init__(
         mode = mode.lower()
         if mode not in self.SUPPORTED_MODES:
             raise ValueError(f"`mode` should be either of {self.SUPPORTED_MODES}")
-        self.optimal_batch_size = init_val
+        self.optimal_batch_size: Optional[int] = init_val
         self._mode = mode
         self._steps_per_trial = steps_per_trial
         self._init_val = init_val