Fix: reassign in qwen3 moe model
Author: linkedlist771
Pull request: yes

Fix: reassign in qwen3 moe model

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Fixes # (issue)

There is reassignment in qwen3 of `self.self_attn` in `Qwen3MoeDecoderLayer`, and the `self.mlp` could be initialized as None.

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Comments:
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hi @linkedlist771, removing the double assignment makes sense, but this PR also sets `self.mlp = None`. Is that intentional?

> hi @linkedlist771, removing the double assignment makes sense, but this PR also sets `self.mlp = None`. Is that intentional?


The PR sets `self.mlp = None`, which is redundant because `self.mlp` is immediately assigned a proper value in the conditional block that follows:
```python
if (layer_idx not in config.mlp_only_layers) and (
    config.num_experts > 0 and (layer_idx + 1) % config.decoder_sparse_step == 0
):
    self.mlp = Qwen3MoeSparseMoeBlock(config)
else:
    self.mlp = Qwen3MoeMLP(config, intermediate_size=config.intermediate_size)
```

The original code had a double assignment issue where it first set `self.mlp = Qwen3MoeMLP(config)` and then potentially reassigned it. This is particularly problematic because initializing these MLP components is computationally expensive in large models - it allocates significant memory and resources that would immediately be discarded when `self.mlp` is reassigned.  We could remove this line or set the `mlp` as None.

Hi @linkedlist771 I think I'd prefer to remove the `None` assignment entirely, since `self.mlp` is assigned after that! I'll make the changes since we want to resolve this fairly soon

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