Raise an exception when extern function does not return Stmt (#5964)

The function for tvm.te.extern should return either PrimExpr or Stmt,
however there is no check if it actually does so. If it does not, the
result may be a segmentation fault later on. Catch this case early on,
so an informative message can be shown.

python/tvm/te/operation.py

@@ -313,6 +313,8 @@ def extern(shape,
     body = fcompute(input_placeholders, output_placeholders)
     if isinstance(body, tvm.tir.PrimExpr):
         body = tvm.tir.Evaluate(body)
+    if not isinstance(body, tvm.tir.Stmt):
+        raise ValueError("Function '{}' should return PrimExpr or Stmt".format(fcompute.__name__))
 
     op = _ffi_api.ExternOp(name, tag, attrs,
                            inputs, input_placeholders,