Why make type convert as a global function? I think the staticmethod is more proper here because we can not call type convert function out of Variable.

The reason this was made a global function is because this is needed outside of the Variable class. In layer_helper, we want to make sure that every parameter which has not been supplied an initializer has a default initializer. This initializer depends on the dtype of the parameter. If the parameter is of type float, then XavierInitializer is used otherwise the parameter is initialized with Zeros for int and bool types.

Now we need this method outside because users can also pass np datatypes as dtypes. The initializer needs to be specified in layer_helper and hence we need to check whether the supplied datatype (which could be np.datatype or core.DataType) is of type float. Do you have any suggestion on how to accomplish this without making this global?

I find a solution resolve this problem. convert_np_dtype_to_dtype_ goes the wrong way...
this function just makes user can configure a data type string float32, float64. But we should only let user configure support datatype like paddle.float32, paddle.float64, and make the real type conversion(from/to numpy) happens in the feed/fetch implementation.

dtype == core.DataType.FP16 should be FP32 here. But I think that we need more general type asserts in c++ side, just throw an exception when the configuration is wrong.

Same as above.