Fix AdaLayerNormZeroSingle.forward return type annotation

[Ricardo-M-L]

What does this PR do?

AdaLayerNormZeroSingle.forward() has a return type annotation declaring a 5-element tuple:

def forward(self, x, emb=None) -> tuple[torch.Tensor, torch.Tensor, torch.Tensor, torch.Tensor, torch.Tensor]:
    ...
    return x, gate_msa  # actually returns 2 elements

The 5-element annotation was copied from AdaLayerNormZero.forward() which does return 5 elements (x, gate_msa, shift_mlp, scale_mlp, gate_mlp), but the "Single" variant only returns 2 (x, gate_msa).

All callers correctly unpack 2 elements (e.g., norm_hidden_states, gate = self.norm(hidden_states, emb=temb) in FluxSingleTransformerBlock), so this is just a type annotation fix.

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• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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Who can review?

@yiyixuxu @sayakpaul