Undefined method `zero_grad_all` in `Tensor.grad_check`

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File: leanpass/tensor.py

The grad_check utility calls self.zero_grad_all(), but the Tensor class only defines zero_grad. This results in an AttributeError when the gradient checker is used, breaking a common debugging workflow.

Fix: replace the call with self.zero_grad() (or implement zero_grad_all to recursively zero gradients of all leaf tensors).

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