Fix: Align forward and backward arguments in cpp_extension tutorial

[ZhigaoyuanWang]

This PR fixes a bug in the lltm_cuda_kernel.cu example by aligning the return values of the lltm_cuda_backward function with the Python backward interface.

In cpp_extension.rst line 1150:

return {d_old_h, d_input, d_weights, d_bias, d_old_cell, d_gates};

the original lltm_cuda_backward function returned six values.

However, the corresponding Python backward interface(line 470) only expects five values to be returned, as it corresponds to the number of input tensors to the forward function:

d_old_h, d_input, d_weights, d_bias, d_old_cell = outputs

This mismatch causes the example to fail at runtime.

This change corrects the lltm_cuda_backward function to return only the five required gradient tensors, bringing it into alignment with the Python side. The d_gates tensor is no longer returned, as its gradient is not needed by the caller.

Before this change: The example code would throw ValueError: too many values to unpack (expected 5).

After this change: The example will run successfully, providing a correct and functional demonstration of a custom CUDA extension.

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/tutorials/3520

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As of commit cea931e with merge base 3b23c0e ():
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[sekyondaMeta]

@zou3519 Can you take a quick look at this?

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