add rec distillation demo

configs/rec/ch_ppocr_v2.0/rec_chinese_lite_train_distillation_v2.1.yml

+Global:
+  debug: false
+  use_gpu: true
+  epoch_num: 800
+  log_smooth_window: 20
+  print_batch_step: 10
+  save_model_dir: ./output/rec_D081
+  save_epoch_step: 3
+  eval_batch_step:
+  - 0
+  - 2000
+  cal_metric_during_train: true
+  pretrained_model: null
+  checkpoints: null
+  save_inference_dir: null
+  use_visualdl: false
+  infer_img: doc/imgs_words/ch/word_1.jpg
+  character_dict_path: ppocr/utils/ppocr_keys_v1.txt
+  character_type: ch
+  max_text_length: 25
+  infer_mode: false
+  use_space_char: false
+  distributed: true
+
+
+Optimizer:
+  name: Adam
+  beta1: 0.9
+  beta2: 0.999
+  lr:
+    name: Cosine
+    learning_rate: 0.0005
+    warmup_epoch: 5
+  regularizer:
+    name: L2
+    factor: 1.0e-05
+Architecture:
+  name: DistillationModel
+  algorithm: Distillation
+  freeze_params:
+  - false
+  - false
+  pretrained: null
+  Models:
+    Student:
+      model_type: rec
+      algorithm: CRNN
+      Transform:
+      Backbone:
+        name: MobileNetV3
+        scale: 0.5
+        model_name: small
+        small_stride: [1, 2, 2, 2]
+      Neck:
+        name: SequenceEncoder
+        encoder_type: rnn
+        hidden_size: 48
+      Head:
+        name: CTCHead
+        fc_decay: 0.00001
+    Teacher:
+      model_type: rec
+      algorithm: CRNN
+      Transform:
+      Backbone:
+        name: MobileNetV3
+        scale: 0.5
+        model_name: small
+        small_stride: [1, 2, 2, 2]
+      Neck:
+        name: SequenceEncoder
+        encoder_type: rnn
+        hidden_size: 48
+      Head:
+        name: CTCHead
+        fc_decay: 0.00001
+  
+
+Loss:
+  name: CombinedLoss
+  loss_config_list:
+  - DistillationCTCLoss:
+      weight: 1.0
+      model_name_list: ["Student", "Teacher"]
+      key: null
+  - DistillationDMLLoss:
+      weight: 1.0
+      model_name_list1: ["Student"]
+      model_name_list2: ["Teacher"]
+
+PostProcess:
+  name: DistillationCTCLabelDecode
+  model_name: "Student"
+  key_out: null
+Metric:
+  name: RecMetric
+  main_indicator: acc
+Train:
+  dataset:
+    name: SimpleDataSet
+    data_dir: ./train_data/
+    label_file_list:
+    - ./train_data/train_list.txt
+    transforms:
+    - DecodeImage:
+        img_mode: BGR
+        channel_first: false
+    - RecAug: null
+    - CTCLabelEncode: null
+    - RecResizeImg:
+        image_shape:
+        - 3
+        - 32
+        - 320
+    - KeepKeys:
+        keep_keys:
+        - image
+        - label
+        - length
+  loader:
+    shuffle: true
+    batch_size_per_card: 128
+    drop_last: true
+    num_sections: 1
+    num_workers: 8
+Eval:
+  dataset:
+    name: SimpleDataSet
+    data_dir: ./train_data
+    label_file_list:
+    - ./train_data/val_list.txt
+    transforms:
+    - DecodeImage:
+        img_mode: BGR
+        channel_first: false
+    - CTCLabelEncode: null
+    - RecResizeImg:
+        image_shape:
+        - 3
+        - 32
+        - 320
+    - KeepKeys:
+        keep_keys:
+        - image
+        - label
+        - length
+  loader:
+    shuffle: false
+    drop_last: false
+    batch_size_per_card: 128
+    num_workers: 8