[NPU] Add fused_linear_cross_entropy operator (#1164)

## Summary
To address the UB overflow issue observed in the benchmark, we
introduced an operator with an NPU-friendly implementation of fused
linear cross entropy. This fused operator relies on several underlying
operations (e.g., large matrix multiplication, softmax, and cross
entropy), so its current benchmark performance is not yet optimal.
Further optimization may be needed.

## Testing Done
Device: Atlas A3
`python -m pytest
./test/transformers/test_fused_linear_cross_entropy.py`
<img width="3270" height="499" alt="image"
src="https://github.com/user-attachments/assets/7f8a63df-f325-43fe-80b9-6268c5f10e29"
/>

- Hardware Type: <BLANK>
- [ ] run `make test` to ensure correctness
- [x] run `make checkstyle` to ensure code style
- [ ] run `make test-convergence` to ensure convergence

Author: lowdy1

src/liger_kernel/ops/backends/_ascend/ops/__init__.py

@@ -26,6 +26,9 @@
 from liger_kernel.ops.backends._ascend.ops.fused_add_rms_norm import LigerFusedAddRMSNormFunction
 from liger_kernel.ops.backends._ascend.ops.fused_add_rms_norm import fused_add_rms_norm_backward
 from liger_kernel.ops.backends._ascend.ops.fused_add_rms_norm import fused_add_rms_norm_forward
+from liger_kernel.ops.backends._ascend.ops.fused_linear_cross_entropy import LigerFusedLinearCrossEntropyFunction
+from liger_kernel.ops.backends._ascend.ops.fused_linear_cross_entropy import fused_linear_cross_entropy_backward
+from liger_kernel.ops.backends._ascend.ops.fused_linear_cross_entropy import fused_linear_cross_entropy_forward
 from liger_kernel.ops.backends._ascend.ops.fused_linear_jsd import LigerFusedLinearJSDFunction
 from liger_kernel.ops.backends._ascend.ops.fused_linear_jsd import fused_linear_jsd_backward
 from liger_kernel.ops.backends._ascend.ops.fused_linear_jsd import fused_linear_jsd_forward
@@ -140,4 +143,7 @@
     "sparsemax_backward",
     "LigerFusedNeighborhoodAttentionFunction",
     "fused_neighborhood_attention_forward",
+    "LigerFusedLinearCrossEntropyFunction",
+    "fused_linear_cross_entropy_forward",
+    "fused_linear_cross_entropy_backward",
 ]