No gradient test in softcap unit test for LigerCrossEntropy

[Tcc0403]

🐛 Describe the bug

Liger-Kernel/test/transformers/test_cross_entropy.py

Lines 188 to 201 in 7e0f459

	_input = _tensor.to(torch.float32).detach().clone().requires_grad_(True)
	_input2 = _tensor.detach().clone().requires_grad_(True)
	target = torch.randint(0, V, (B * T,), device=device, dtype=torch.long)
	# downcasting to original dtype
	output = torch_ce(softcap * torch.tanh(_input / softcap), target).to(dtype)
	output2 = target_ce(_input2, target)
	assert torch.allclose(output, output2, atol=atol, rtol=rtol)
	output.backward()
	output2.backward()

There should be a torch.allclose() after backward().

to.(torch.float32) in L188 can be moved to L194

Reproduce

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[austin362667]

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