[ONNX]Remove cond_var from Input('X') in while_loop op (#45675)

* [ONXX]Remove cond_var from Input(X) in while_loop op

* fix unittest

python/paddle/fluid/layers/control_flow.py

@@ -1162,6 +1162,9 @@ class While(object):
                 out_vars.append(inner_var)
 
         x_name_list |= set(map(lambda x: x.name, out_vars))
+        # NOTE(dev): cond_var has been contained in Input('Condition'), so
+        # we remove it from Input('X')
+        x_name_list -= {self.cond_var.name}
 
         step_scope = parent_block.create_var(
             type=core.VarDesc.VarType.STEP_SCOPES)

python/paddle/fluid/tests/unittests/test_while_op.py

@@ -223,6 +223,7 @@ class TestOutputsMustExistsInputs(unittest.TestCase):
         for op in main_program.block(0).ops:
             if op.type == "while":
                 for out_name in op.output("Out"):
+                    if out_name in op.input("Condition"): continue
                     self.assertTrue(
                         out_name in op.input("X"),
                         "In while op, the variable in output(`Out`) must exists in inputs(`X`), but the variable with name `{}` not meet the precondition."