Add inplace=False to liger_cross_entropy to fix upstream gradient corruption (#272)

[lollinng]

Problem (#272)

liger_cross_entropy stores its backward gradient in-place into its input tensor via a Triton tl.store. Triton writes don't bump PyTorch's tensor version counter, so autograd's in-place-correctness check never fires. When the input is the output of an upstream op (e.g. a softmax) that saved it for its own backward, that tensor is silently overwritten and the upstream op computes wrong gradients with no error — the exact scenario in #272.

Fix

Thread an inplace flag (default True, so existing behavior and the memory savings are unchanged) through liger_cross_entropy → LigerCrossEntropyFunction → cross_entropy_forward. When inplace=False, the gradient is computed into a clone of the input, leaving the caller's tensor intact.

One subtlety worth calling out: the clone happens inside autograd.Function.forward (grad disabled), so the clone reports requires_grad=False. Since the kernel only writes the gradient when HAS_GRADIENTS is true, I capture requires_grad from the original input before cloning — otherwise the kernel would skip the gradient write entirely.

Verification (NVIDIA T4)

Ran the issue's reproducer (softmax(_p) → cross_entropy → backward) on a GPU, comparing the gradient w.r.t. the pre-softmax input against F.cross_entropy:

reference (F.cross_entropy) _p.grad[0]: [ 0.00168,  0.00287,  0.00029,  0.00546, -0.01822,  0.00242,  0.00231,  0.00319]
liger default (in-place)    _p.grad[0]: [ 2.1e-05,  3.5e-05,  6.8e-06,  6.7e-05,  0.01325,  3.0e-05,  2.8e-05,  3.9e-05]  -> matches ref: False  (the bug)
liger inplace=False         _p.grad[0]: [ 0.00168,  0.00287,  0.00029,  0.00546, -0.01822,  0.00242,  0.00231,  0.00319]  -> matches ref: True
loss matches ref: True

Added test/transformers/test_cross_entropy.py::test_cross_entropy_inplace_does_not_corrupt_upstream_grad, which asserts the default path corrupts the upstream gradient (reproducing the bug) and that inplace=False matches the F.cross_entropy reference.

Fixes #272