Fix Op (Slice complex) | improve dim expression (#2094)

Author: titaiwangms

onnxscript/function_libs/torch_lib/ops/core.py

@@ -7709,7 +7709,7 @@ def aten_slice_complex(
     """slice.Tensor(Tensor(a) self, int dim=0, SymInt? start=None, SymInt? end=None, SymInt step=1) -> Tensor(a)"""
     if dim < 0:
         # Account for the complex dimension in ONNX
-        dim = dim - 1
+        dim = len(self.shape) + dim - 1
     return aten_slice(self, dim, start, end, step)