[Fix] Improve MultiImageMixDataset unittests coverage (open-mmlab#1197)

* Fix typo in usage example

* original MultiImageMixDataset code in mmdet

* Add MultiImageMixDataset unittests in test_dataset_wrapper

* fix lint error

* fix value name ann_file to ann_dir

* modify retrieve_data_cfg (open-mmlab#1)

* remove dynamic_scale & add palette

* modify retrieve_data_cfg method

* modify retrieve_data_cfg func

* fix error

* improve the unittests coverage

* fix unittests error

* Dataset (open-mmlab#2)

* add cfg-options

* Add unittest in test_build_dataset

* add blank line

* add blank line

* add a blank line

Co-authored-by: Miao Zheng <76149310+MeowZheng@users.noreply.github.com>

* [Fix] Add MultiImageMixDataset unittests

Co-authored-by: Younghoon-Lee <72462227+Younghoon-Lee@users.noreply.github.com>
Co-authored-by: MeowZheng <meowzheng@outlook.com>
Co-authored-by: Miao Zheng <76149310+MeowZheng@users.noreply.github.com>

tests/test_data/test_dataset.py

@@ -105,8 +105,7 @@ def test_dataset_wrapper():
 
     img_scale = (60, 60)
     pipeline = [
-        # dict(type='Mosaic', img_scale=img_scale, pad_val=255),
-        # need to merge mosaic
+        dict(type='RandomMosaic', prob=1, img_scale=img_scale),
         dict(type='RandomFlip', prob=0.5),
         dict(type='Resize', img_scale=img_scale, keep_ratio=False),
     ]
@@ -130,14 +129,8 @@ def test_dataset_wrapper():
         classes=classes,
         palette=palette)
     len_a = 2
-    cat_ids_list_a = [
-        np.random.randint(0, 80, num).tolist()
-        for num in np.random.randint(1, 20, len_a)
-    ]
-    dataset_a.data_infos = MagicMock()
-    dataset_a.data_infos.__len__.return_value = len_a
-    dataset_a.get_cat_ids = MagicMock(
-        side_effect=lambda idx: cat_ids_list_a[idx])
+    dataset_a.img_infos = MagicMock()
+    dataset_a.img_infos.__len__.return_value = len_a
 
     multi_image_mix_dataset = MultiImageMixDataset(dataset_a, pipeline)
     assert len(multi_image_mix_dataset) == len(dataset_a)