[torchlib] Fix aten_gather to correctly handle scalar indices (#2566)

Fixes #2564

Signed-off-by: Linsho Kaku <linsho@preferred.jp>

Author: linshokaku

onnxscript/function_libs/torch_lib/ops/core.py

@@ -3814,11 +3814,15 @@ def aten_gather(
         else:
             return op.Expand(self, op.Shape(index))
 
-    if len(index.shape) == 0:
-        return op.Identity(self)
+    is_scalar_index = len(index.shape) == 0
+    if is_scalar_index:
+        index = op.Unsqueeze(index, [0])
 
     index = op.Cast(index, to=INT64.dtype)
     result = op.GatherElements(self, index, axis=dim)
+
+    if is_scalar_index:
+        result = op.Squeeze(result, [0])
     return result