[Fix] Add swap_labe_pairs in RandomFlip

[MeowZheng]

Motivation

To handle datasets with left/right annotations when flip the seg map horizontally.

Modification

Add swap_label_pairs in RandomFlip init, and swap the labels of a seg map when swap_label_pairs is defined.

BC-breaking (Optional)

No

Use cases (Optional)

LIP dataset data augmentation

Checklist

Before PR:

• I have read and followed the workflow indicated in the CONTRIBUTING.md to create this PR.
• Pre-commit or linting tools indicated in CONTRIBUTING.md are used to fix the potential lint issues.
• 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.
• The documentation has been modified accordingly, including docstring or example tutorials.

After PR:

• If the modification has potential influence on downstream or other related projects, this PR should be tested with some of those projects, like MMDet or MMCls.
• CLA has been signed and all committers have signed the CLA in this PR.

[zhouzaida]

Suggested change

	def _flip_keypoints(self, keypoints: np.ndarray, img_shape: Tuple[int,
	int],
	direction: str) -> np.ndarray:
	def _flip_keypoints(
	self,
	keypoints: np.ndarray,
	img_shape: Tuple[int, int],
	direction: str,) -> np.ndarray:

[mzr1996]

Does the swap_label_paris only affect seg maps? How about key points?

[mzr1996]

And better to provide the detailed usage of the argument. How to specify the left arm and right arm pair?

[mzr1996]

Better to add an example here, like

Suggested change

	swap_seg_labels (list, optional): The label pair need to be swapped
	for ground truth, like 'left arm' and 'right arm' need to be
	swapped after horizontal flipping.
	swap_seg_labels (list, optional): The label pair need to be swapped
	for ground truth, like 'left arm' and 'right arm' need to be
	swapped after horizontal flipping. For example, ``[(1, 5)]``,
	where 1/5 is the label of the left/right arm. Defaults to None.