update docs

docs/build_task.md

@@ -103,4 +103,23 @@ def build_model(cls, args, train):
         )
     return model
 ```
-This function defines the detail of the model. For different speech recognition models, the same speech recognition `Task` can usually be shared and the remaining thing needed to be done is to define a specific model in this function. For example, a speech recognition model with a standard encoder-decoder structure has been shown above. Specifically, it first defines each module of the model, including encoder, decoder, etc. and then combine these modules together to generate a complete model. In FunASR, the model needs to inherit `AbsESPnetModel` and the corresponding code can be seen in `funasr/train/abs_espnet_model.py`. The main function needed to be implemented is the `forward` function.
+This function defines the detail of the model. For different speech recognition models, the same speech recognition `Task` can usually be shared and the remaining thing needed to be done is to define a specific model in this function. For example, a speech recognition model with a standard encoder-decoder structure has been shown above. Specifically, it first defines each module of the model, including encoder, decoder, etc. and then combine these modules together to generate a complete model. In FunASR, the model needs to inherit `AbsESPnetModel` and the corresponding code can be seen in `funasr/train/abs_espnet_model.py`. The main function needed to be implemented is the `forward` function.
+
+Next, we take `SANMEncoder` as an example to introduce how to use a custom encoder as a part of the model when defining the specified model and the corresponding code can be seen in `funasr/models/encoder/sanm_encoder.py`. For a custom encoder, in addition to inheriting the common encoder class `AbsEncoder`, it is also necessary to define the `forward` function to achieve the forward computation of the `encoder`. After defining the `encoder`, it should also be registered in the `Task`. The corresponding code example can be seen as below:
+```python
+encoder_choices = ClassChoices(
+    "encoder",
+    classes=dict(
+        conformer=ConformerEncoder,
+        transformer=TransformerEncoder,
+        rnn=RNNEncoder,
+        sanm=SANMEncoder,
+        sanm_chunk_opt=SANMEncoderChunkOpt,
+        data2vec_encoder=Data2VecEncoder,
+        mfcca_enc=MFCCAEncoder,
+    ),
+    type_check=AbsEncoder,
+    default="rnn",
+)
+```
+In this code, `sanm=SANMEncoder` takes the newly defined `SANMEncoder` as an optional choice of the `encoder`. Once the user specifies the `encoder` as `sanm` in the configuration file, the `SANMEncoder` will be correspondingly employed as the `encoder` module of the model.