Fix SlicesSplit rewrite rule (#2802)

The logic was incorrectly trying access an attribute that does not
exist.

---------

Signed-off-by: Justin Chu <justinchuby@users.noreply.github.com>

Author: justinchuby

onnxscript/rewriter/rules/common/_basic_rules.py

@@ -178,11 +178,11 @@ def check(self, context, x, begin0, end0, axes0, begin1, end1, axes1) -> MatchRe
         axes = axes0.const_value.numpy().tolist()
         if len(axes) != 1:
             return check_result.fail("Axes has more than one dimension.")
-        if x.shape:
-            rk = len(x.shape)
+        if x.shape is not None:
+            rank = len(x.shape)
         else:
-            rk = x.rank
-        if axes[0] != -1 and axes[0] != rk - 1:
+            return check_result.fail("Input rank is not known.")
+        if axes[0] != -1 and axes[0] != rank - 1:
             return check_result.fail("Axes is not -1 or last dimension.")
         if (
             begin0.const_value is None