fix mypy issues

Author: topper-123

Diff for: pandas/core/apply.py

@@ -227,7 +227,7 @@ def transform(self) -> DataFrame | Series:
             func = cast(AggFuncTypeDict, func)
             return self.transform_dict_like(func)
 
-        if not self.by_row:
+        if not by_row:
             result = obj.apply(func, by_row=by_row, args=args, **kwargs)
         else:
             # func is either str or callable

Diff for: pandas/core/frame.py

@@ -9921,7 +9921,7 @@ def transform(
                 stacklevel=find_stack_level(),
             )
 
-        by_row = False if series_ops_only else "compat"
+        by_row: Literal[False, "compat"] = False if series_ops_only else "compat"
 
         op = frame_apply(
             self, func=func, axis=axis, args=args, by_row=by_row, kwargs=kwargs

Diff for: pandas/core/series.py

@@ -4636,7 +4636,7 @@ def transform(
                 stacklevel=find_stack_level(),
             )
 
-        by_row = False if series_ops_only else "compat"
+        by_row: Literal[False, "compat"] = False if series_ops_only else "compat"
         ser = self.copy(deep=False) if using_copy_on_write() else self
         result = SeriesApply(
             ser, func=func, by_row=by_row, args=args, kwargs=kwargs