GELU  
GELU activation function with tanh approximation support with `approximate="tanh"`.  
Parameters:
dim_in (`int`): The number of channels in the input.
dim_out (`int`): The number of channels in the output.
approximate (`str`, *optional*, defaults to `"none"`): If `"tanh"`, use tanh approximation.
bias (`bool`, defaults to True): Whether to use a bias in the linear layer.