Qwen3-4B-Thinking-2509-Genius-Coder-AI-Full

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<|im_start|>user
write a python code using pytorch for a simple neural network demo?

Please provide step-by-step reasoning inside <think> tags, AND then output the final Python code after </think>.<|im_end|>
<|im_start|>assistant
<think>
**Planning the step-by-step**

I need to create a simple PyTorch neural network. The plan is: import torch and torch.nn; define a neural network with one input layer, one hidden layer (ReLU activation), one output layer, set up random data, train the network using gradient descent and a loss function (MSE), evaluate its performance on new data. Code will be written in a single script following this logic.
</think>