update pre-commit hook config (#384)

.pre-commit-config.yaml

 exclude: ^tests/data/
 repos:
   - repo: https://gitlab.com/pycqa/flake8.git
-    rev: 3.8.0
+    rev: 3.8.3
     hooks:
       - id: flake8
   - repo: https://github.com/asottile/seed-isort-config
-    rev: v2.1.0
+    rev: v2.2.0
     hooks:
       - id: seed-isort-config
   - repo: https://github.com/timothycrosley/isort
@@ -17,13 +17,14 @@ repos:
     hooks:
       - id: yapf
   - repo: https://github.com/pre-commit/pre-commit-hooks
-    rev: v2.5.0
+    rev: v3.1.0
     hooks:
       - id: trailing-whitespace
       - id: check-yaml
       - id: end-of-file-fixer
       - id: requirements-txt-fixer
       - id: double-quote-string-fixer
+      - id: check-merge-conflict
       - id: fix-encoding-pragma
         args: ["--remove"]
       - id: mixed-line-ending

mmcv/ops/wrappers.py

-"""Modified from https://github.com/facebookresearch/detectron2/blob/master.
+r"""Modified from https://github.com/facebookresearch/detectron2/blob/master/detectron2/layers/wrappers.py  # noqa: E501
 
-/detectron2/layers/wrappers.py Wrap some nn modules to support empty
-tensor input. Currently, these wrappers are mainly used in mask heads
-like fcn_mask_head and maskiou_heads since mask heads are trained on
-only positive RoIs.
+Wrap some nn modules to support empty tensor input. Currently, these wrappers
+are mainly used in mask heads like fcn_mask_head and maskiou_heads since mask
+heads are trained on only positive RoIs.
 """
 import math