[Hybrid][torch.compile] Refactor mamba2 forward to avoid obscuring linear projections under custom op (#28587)

Signed-off-by: Tomer Asida <57313761+tomeras91@users.noreply.github.com>

vllm/model_executor/models/granitemoehybrid.py

@@ -115,8 +115,7 @@ class GraniteMoeHybridMambaDecoderLayer(nn.Module):
     ):
         residual = hidden_states
         hidden_states = self.input_layernorm(hidden_states)
-        output = torch.empty_like(hidden_states)
-        self.mamba(hidden_states, output)
+        output = self.mamba(hidden_states)
         hidden_states = residual + output * self.residual_multiplier
 
         residual = hidden_states