add developing doc

Signed-off-by: Jinzhe Zeng <[email protected]>

doc/development/create-a-model-pt.md

@@ -137,6 +137,15 @@ class SomeAtomicModel(BaseAtomicModel, torch.nn.Module):
         pass
 ```
 
+### Floating-point precision
+
+When creating a new component, the floating-point precision should obey the [Floating-point precision of the model](../model/precision.md) section.
+In implementation, the component should
+
+- store parameters in the component precision, except those for input and output normalization;
+- store normalization parameters in {py:data}`deepmd.pt.utils.env.GLOBAL_PT_FLOAT_PRECISION`;
+- after input normalization, cast the input tensor to the component precision; before output normalization, cast the output tensor to the {py:data}`deepmd.pt.utils.env.GLOBAL_PT_FLOAT_PRECISION`.
+
 ## Register new arguments
 
 To let someone uses your new component in their input file, you need to create a new method that returns some `Argument` of your new component, and then register new arguments. For example, the code below