def get_activation_index_and_reconstitute_activation_fn(
    transformer: Transformer,
    multi_autoencoder_context: MultiAutoencoderContext | None,
    trace_config: TraceConfig,
) -> tuple[ActivationIndex, Callable[[torch.Tensor, torch.Tensor | None], torch.Tensor]]:
    """
    This function returns the ActivationIndex corresponding to the preceding residual stream
    index implied by the trace_config. It also returns a function taking one tensor, used to recompute
    the activation specified by the trace_config from the residual stream.
    """
    assert trace_config.attention_trace_type != AttentionTraceType.V
    if trace_config.node_type.is_autoencoder_latent:
        assert multi_autoencoder_context is not None
        autoencoder_context = multi_autoencoder_context.get_autoencoder_context(
            trace_config.node_type
        )
        assert autoencoder_context is not None
    else:
        autoencoder_context = None
    act_reconstituter = ActivationReconstituter.from_trace_config(
        transformer=transformer,
        autoencoder_context=autoencoder_context,
        trace_config=trace_config,
    )
    activation_index_for_reconstituter = act_reconstituter.get_residual_activation_index_for_node_index(
        # convert trace_config to node index
        trace_config.node_index
    )

    def reconstitute_activation_fn(
        upstream_resid: torch.Tensor, _unused_downstream_resid: torch.Tensor | None
    ) -> torch.Tensor:
        assert _unused_downstream_resid is None
        reconstitute_activation = (
            act_reconstituter.make_reconstitute_activation_fn_for_trace_config(
                trace_config=trace_config
            )
        )
        return reconstitute_activation(upstream_resid)

    return activation_index_for_reconstituter, reconstitute_activation_fn

    This function returns the ActivationIndex corresponding to the preceding residual stream
    index implied by the trace_config. It also returns a function taking one tensor, used to recompute
    the activation specified by the trace_config from the residual stream.
    
neuron_explainer/activation_server/derived_scalar_computation.py