keras/src/layers/rnn/simple_rnn.py/SimpleRNNCell/build
class SimpleRNNCell:    def build(self, input_shape):
        self.kernel = self.add_weight(
            shape=(input_shape[-1], self.units),
            name="kernel",
            initializer=self.kernel_initializer,
            regularizer=self.kernel_regularizer,
            constraint=self.kernel_constraint,
        )
        self.recurrent_kernel = self.add_weight(
            shape=(self.units, self.units),
            name="recurrent_kernel",
            initializer=self.recurrent_initializer,
            regularizer=self.recurrent_regularizer,
            constraint=self.recurrent_constraint,
        )
        if self.use_bias:
            self.bias = self.add_weight(
                shape=(self.units,),
                name="bias",
                initializer=self.bias_initializer,
                regularizer=self.bias_regularizer,
                constraint=self.bias_constraint,
            )
        else:
            self.bias = None
        self.built = True