[Feature] TextRecogCropConverter add crop with opencv warpPersepective function

[KevinNuNu]

…e function.

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Motivation

自定义数据集中会有一些由于拍摄角度以及透视等问题造成的文字区域倾斜或者旋转，在标注时会用到polygon的标注方式。同时密集文本场景，不适合将polygon标注转换为水平文本框进行裁剪，需要用opencv warpPersepective方法进行裁剪

Modification

1.优化TextRecogCropConverter的pack_instance()方法，增加crop_with_warp选择按钮，引入mmocr/utils/img_utils.py中的warp_img方法，完成四边形标注的旋转文本区域的裁剪功能
2.完善了一下相应的使用文档：docs/zh_cn/user_guides/data_prepare/dataset_preparer.md

BC-breaking (Optional)

Does the modification introduce changes that break the backward-compatibility of the downstream repositories?
If so, please describe how it breaks the compatibility and how the downstream projects should modify their code to keep compatibility with this PR.

Use cases (Optional)

If this PR introduces a new feature, it is better to list some use cases here, and update the documentation.

Checklist

Before PR:

• I have read and followed the workflow indicated in the CONTRIBUTING.md to create this PR.
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• Bug fixes are covered by unit tests, the case that causes the bug should be added in the unit tests.
• New functionalities are covered by complete unit tests. If not, please add more unit test to ensure the correctness.
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[gaotongxiao]

Nice feature!

[gaotongxiao]

Suggested change

	if 'box' in instance:
	return bbox2poly(instance['box']).tolist()
	if 'poly' in instance:
	return instance['poly']
	if 'poly' in instance:
	return instance['poly']
	if 'box' in instance:
	return bbox2poly(instance['box']).tolist()

box is just a rough approximation of polygon, so polygon should be the first choice