Expose bias to to ModulesToSaveWrapper

[dengdifan]

Similar to #1238 and #1530

This PR aims at solving an issue similar to #1524 and #1225, where one would like to add self.dt_proj module to modules_to_save instead of target_modules for the following operation

            contextualized_states = mamba_inner_fn(
                projected_states,
                self.conv1d.weight,
                self.conv1d.bias if self.use_conv_bias else None,
                self.x_proj.weight,
                self.dt_proj.weight,
                self.out_proj.weight,
                self.out_proj.bias.float() if self.use_bias else None,
                -torch.exp(self.A_log.float()),
                None,  # input-dependent B
                None,  # input-dependent C
                self.D.float(),
                delta_bias=self.dt_proj.bias.float(),
                delta_softplus=True,
            )

[HuggingFaceDocBuilderDev]

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[BenjaminBossan]

Thanks for adding this feature and not forgetting to extend the tests.