Add missing ignore_index tests (#310)

## Summary
`ignore_index` in fused_linear_cross_entropy was not tested

## Testing Done

- Hardware Type: gpu-ci
- [x] run `make test` to ensure correctness
- [x] run `make checkstyle` to ensure code style
- [x] run `make test-convergence` to ensure convergence

---------

Co-authored-by: Byron Hsu <byronhsu1230@gmail.com>
Co-authored-by: Yun Dai <yundai424@gmail.com>

test/transformers/test_fused_linear_cross_entropy.py

@@ -100,16 +100,28 @@ def forward(self, x, y):
     ],
 )
 @pytest.mark.parametrize("bias", [True, False])
-@pytest.mark.parametrize("label_smoothing", [0, 0.1])
+@pytest.mark.parametrize("label_smoothing, ignore_index", [(0.0, -100), (0.1, 42)])
 def test_correctness(
-    B, T, H, V, scalar, dtype, bias, label_smoothing, reduction, atol, rtol
+    B,
+    T,
+    H,
+    V,
+    scalar,
+    dtype,
+    bias,
+    label_smoothing,
+    ignore_index,
+    reduction,
+    atol,
+    rtol,
 ):
     device = "cuda"
     torch_lm_head_ce = TorchLMHeadCE(
         H=H,
         V=V,
         bias=bias,
         label_smoothing=label_smoothing,
+        ignore_index=ignore_index,
         reduction=reduction,
         dtype=dtype,
     ).to(device)
@@ -118,6 +130,7 @@ def test_correctness(
         V=V,
         bias=bias,
         label_smoothing=label_smoothing,
+        ignore_index=ignore_index,
         reduction=reduction,
         dtype=dtype,
     ).to(device)
@@ -137,6 +150,14 @@ def test_correctness(
     _input2 = _tensor.detach().clone().requires_grad_(True)
 
     target = torch.randint(0, V, (B * T,), device=device, dtype=torch.long)
+    # Assign some random number of elements as ignore_index
+    num_elements_to_assign = torch.randint(
+        1, B * T // 2, (1,)
+    ).item()  # Random number of elements to set to ignore_index
+    indices_to_assign = torch.randperm(B * T)[
+        :num_elements_to_assign
+    ]  # Randomly select indices
+    target[indices_to_assign] = ignore_index
 
     output1 = torch_lm_head_ce(_input1, target)
     output2 = liger_lm_head_ce(_input2, target)