Fix parser error and add explain examples (#2538)

### What problem does this PR solve?

1. Fix explain error option bug
2. Add explain python examples.

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

Signed-off-by: Jin Hai <haijin.chn@gmail.com>

example/simple_example.py

@@ -18,6 +18,7 @@
 
 # import infinity_embedded as infinity
 import infinity
+from infinity.table import ExplainType
 import sys
 
 try:
@@ -66,6 +67,9 @@
     if extra_result is not None:
         print(extra_result)
 
+    res = table_instance.output(["num", "body", "vec"]).explain(ExplainType.Pipeline)
+    print(res)
+
     infinity_instance.disconnect()
 
     print('test done')