[Fix] Update the ClassBalancedDataset logic

[BIGWangYuDong]

Related PR: open-mmlab/mmdetection#10038

Description of the problem

Right now, if the cat_ids is empty, the operation will be skipped. And the length of the repeat_factors will not equal to len(self.dataset).

mmengine/mmengine/dataset/dataset_wrapper.py

Lines 430 to 437 in 093068e

	repeat_factors = []
	for idx in range(num_images):
	cat_ids = set(self.dataset.get_cat_ids(idx))
	if len(cat_ids) != 0:
	repeat_factor = max(
	{category_repeat[cat_id]
	for cat_id in cat_ids})
	repeat_factors.append(repeat_factor)

The index of the image without any label will be replaced by the next image, and this will cause error.

mmengine/mmengine/dataset/dataset_wrapper.py

Lines 383 to 391 in 093068e

	repeat_factors = self._get_repeat_factors(self.dataset,
	self.oversample_thr)
	# Repeat dataset's indices according to repeat_factors. For example,
	# if `repeat_factors = [1, 2, 3]`, and the `len(dataset) == 3`,
	# the repeated indices will be [1, 2, 2, 3, 3, 3].
	repeat_indices = []
	for dataset_index, repeat_factor in enumerate(repeat_factors):
	repeat_indices.extend([dataset_index] * math.ceil(repeat_factor))
	self.repeat_indices = repeat_indices

Hence, if the image label is empty, it should add1.0 in repeat_factors to keep the length same.

Results of LVIS

Before fix this error

 Epoch(val) [12][2477/2477]  
lvis/bbox_AP: 0.2160  lvis/bbox_AP50: 0.3520  lvis/bbox_AP75: 0.2290  lvis/bbox_APs: 0.1660  lvis/bbox_APm: 0.2860  lvis/bbox_APl: 0.3350  lvis/bbox_APr: 0.0630  lvis/bbox_APc: 0.2000  lvis/bbox_APf: 0.3020 
lvis/segm_AP: 0.2080  lvis/segm_AP50: 0.3290  lvis/segm_AP75: 0.2210  lvis/segm_APs: 0.1510  lvis/segm_APm: 0.2830  lvis/segm_APl: 0.3390  lvis/segm_APr: 0.0660  lvis/segm_APc: 0.2010  lvis/segm_APf: 0.2790

After fix this error

 Epoch(val) [12][2477/2477]  
lvis/bbox_AP: 0.2300  lvis/bbox_AP50: 0.3770  lvis/bbox_AP75: 0.2430  lvis/bbox_APs: 0.1730  lvis/bbox_APm: 0.3030  lvis/bbox_APl: 0.3460  lvis/bbox_APr: 0.1000  lvis/bbox_APc: 0.2170  lvis/bbox_APf: 0.3010  
lvis/segm_AP: 0.2240  lvis/segm_AP50: 0.3520  lvis/segm_AP75: 0.2380  lvis/segm_APs: 0.1590  lvis/segm_APm: 0.3000  lvis/segm_APl: 0.3520  lvis/segm_APr: 0.1180  lvis/segm_APc: 0.2160  lvis/segm_APf: 0.2790

Released result in mmdet

{"mode": "val", "epoch": 12, "iter": 7674, "lr": 0.0002,
 "bbox_AP": 0.225, "bbox_AP50": 0.369, "bbox_AP75": 0.238, "bbox_APs": 0.168, "bbox_APm": 0.297, "bbox_APl": 0.35, "bbox_APr": 0.091, "bbox_APc": 0.211, "bbox_APf": 0.301, 
 "bbox_mAP_copypaste": "AP:0.225 AP50:0.369 AP75:0.238 APs:0.168 APm:0.297 APl:0.350 APr:0.091 APc:0.211 APf:0.301", 
 "segm_AP": 0.217, "segm_AP50": 0.343, "segm_AP75": 0.23, "segm_APs": 0.152, "segm_APm": 0.294, "segm_APl": 0.351, "segm_APr": 0.096, "segm_APc": 0.21, "segm_APf": 0.278, 
 "segm_mAP_copypaste": "AP:0.217 AP50:0.343 AP75:0.230 APs:0.152 APm:0.294 APl:0.351 APr:0.096 APc:0.210 APf:0.278"}

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