[Docs] Add tutorial of dataset

docs/en/tutorials/customize_dataset.md

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+# Tutorial: Customize Datasets
+
+## Supported Data Format
+
+### Image Super-Resolution
+
+- [SRAnnotationDataset](/mmedit/datasets/sr_annotation_dataset.py)
+  General paired image dataset with an annotation file for image restoration.
+- [SRFolderDataset](/mmedit/datasets/sr_folder_dataset.py)
+  General paired image folder dataset for image restoration.
+- [SRFolderGTDataset](/mmedit/datasets/sr_folder_gt_dataset.py)
+  General ground-truth image folder dataset for image restoration, where low-quality image should be generated in pipeline.
+- [SRFolderRefDataset](/mmedit/datasets/sr_folder_ref_dataset.py)
+  General paired image folder dataset for reference-based image restoration.
+- [SRLmdbDataset](/mmedit/datasets/sr_lmdb_dataset.py)
+  General paired image lmdb dataset for image restoration.
+- [SRFacialLandmarkDataset](/mmedit/datasets/sr_facial_landmark_dataset.py)
+  Facial image and landmark dataset with an annotation file.
+
+### Video Super-Resolution
+
+- [SRFolderMultipleGTDataset](/mmedit/datasets/sr_folder_multiple_gt_dataset.py)
+  General dataset for video super resolution, used for recurrent networks.
+- [SRREDSDataset](/mmedit/datasets/sr_reds_dataset.py)
+  REDS dataset for video super resolution.
+- [SRREDSMultipleGTDataset](/mmedit/datasets/sr_reds_multiple_gt_dataset.py)
+  REDS dataset for video super resolution for recurrent networks.
+- [SRTestMultipleGTDataset](/mmedit/datasets/sr_test_multiple_gt_dataset.py)
+  Test dataset for video super resolution for recurrent networks.
+- [SRVid4Dataset](/mmedit/datasets/sr_vid4_dataset.py)
+  Vid4 dataset for video super resolution.
+- [SRVimeo90KDataset](/mmedit/datasets/sr_vimeo90k_dataset.py)
+  Vimeo90K dataset for video super resolution.
+- [SRVimeo90KMultipleGTDataset](/mmedit/datasets/sr_vimeo90k_multiple_gt_dataset.py)
+  Vimeo90K dataset for video super resolution for recurrent networks.
+
+### Video Frame Interpolation
+
+- [VFIVimeo90KDataset](/mmedit/datasets/vfi_vimeo90k_dataset.py)
+  Vimeo90K dataset for video frame interpolation.
+
+### Matting
+
+- [AdobeComp1kDataset](/mmedit/datasets/comp1k_dataset.py)
+  Adobe composition-1k dataset.
+
+### Inpainting
+
+- [ImgInpaintingDataset](/mmedit/datasets/img_inpainting_dataset.py)
+  Only use the image name information from annotation file.
+
+### Generation
+
+- [GenerationPairedDataset](/mmedit/datasets/generation_paired_dataset.py)
+  General paired image folder dataset for image generation.
+- [GenerationUnpairedDataset](/mmedit/datasets/generation_unpaired_dataset.py)
+  General unpaired image folder dataset for image generation.
+
+## Support new data format
+
+You can reorganize new data formats to existing format.
+
+Or create a new dataset in [mmedit/datasets](/mmedit/datasets) to load the data. 
+
+Inheriting from the base class of datasets will make it easier to create a new dataset
+- [BaseSRDataset](/mmedit/datasets/base_sr_dataset.py)
+- [BaseVFIDataset](/mmedit/datasets/base_vfi_dataset.py)
+- [BaseMattingDataset](/mmedit/datasets/base_matting_dataset.py)
+- [BaseGenerationDataset](/mmedit/datasets/base_generation_dataset.py)
+
+If you want create a dataset for a new low level CV task (e.g. denoise, derain, defog, and de-reflection), you can inheriting from [BaseDataset](/mmedit/datasets/base_dataset.py). 
+
+Welcome to [submit new dataset classes to MMEditing](https://github.com/open-mmlab/mmediting/compare).
+
+Here is an example of create a dataset for video frame interpolation:
+
+```python
+import os
+import os.path as osp
+
+from .base_vfi_dataset import BaseVFIDataset
+from .registry import DATASETS
+
+
+@DATASETS.register_module()
+class NewVFIDataset(BaseVFIDataset):
+    """Introduce the dataset
+
+    Examples of file structure.
+
+    Args:
+        pipeline (list[dict | callable]): A sequence of data transformations.
+        folder (str | :obj:`Path`): Path to the folder.
+        ann_file (str | :obj:`Path`): Path to the annotation file.
+        test_mode (bool): Store `True` when building test dataset.
+            Default: `False`.
+    """
+
+    def __init__(self, pipeline, folder, ann_file, test_mode=False):
+        super().__init__(pipeline, folder, ann_file, test_mode)
+        self.data_infos = self.load_annotations()
+
+    def load_annotations(self):
+        """Load annoations for the dataset.
+
+        Returns:
+            list[dict]: A list of dicts for paired paths and other information.
+        """
+        data_infos = []
+        ...
+        return data_infos
+
+```
+
+## Customize datasets by dataset wrappers
+
+### Repeat dataset
+
+We use [RepeatDataset](mmedit/datasets/dataset_wrappers.py) as wrapper to repeat the dataset.
+For example, suppose the original dataset is Dataset_A, to repeat it, the config looks like the following
+
+```python
+dataset_A_train = dict(
+        type='RepeatDataset',
+        times=N,
+        dataset=dict(  # This is the original config of Dataset_A
+            type='Dataset_A',
+            ...
+            pipeline=train_pipeline
+        )
+    )
+```