[pre-commit.ci] auto fixes from pre-commit.com hooks

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Author: pre-commit-ci[bot]

monai/losses/focal_loss.py

@@ -12,7 +12,8 @@
 from __future__ import annotations
 
 import warnings
-from typing import Optional, Sequence
+from typing import Optional
+from collections.abc import Sequence
 
 import torch
 import torch.nn.functional as F