`Tensor.grad_check` calls undefined `zero_grad_all` method

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File: leanpass/tensor.py

During gradient checking the code invokes self.zero_grad_all(), but Tensor only defines zero_grad. This results in an AttributeError and makes grad_check unusable. Replace the call with the existing method or implement zero_grad_all that zeros gradients of all leaf tensors.

# replace
- self.zero_grad_all()
+ self.zero_grad()

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