First wiki page

How-to-train-a-model?.md

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+1. Define a model, which is initialized with `args`. 
+[example](https://github.com/fastnlp/fastNLP/blob/743a6d75474fa393bc849c0392317c2c6bd868e4/fastNLP/models/sequence_modeling.py#L12)
+
+2. Set model parameters in the config file. The parameters you set will be packed as `args` and used in model initialization. 
+[example](https://github.com/fastnlp/fastNLP/blob/743a6d75474fa393bc849c0392317c2c6bd868e4/reproduction/chinese_word_seg/cws.cfg#L1)
+
+3. Write a script to use Trainer. 
+[example](https://github.com/fastnlp/fastNLP/blob/743a6d75474fa393bc849c0392317c2c6bd868e4/reproduction/chinese_word_seg/cws_train.py#L51)