some fixes

[zhiics]

Some fixes. I tested it locally test_vm and test_vm_serialization can all pass. CI should be happy now. Two other fixes. The first one is the serialization.

In the second one, I removed the last constant folding pass in memory pass to make test_vm_serialization pass (like MobileNet and ResNet). It actually reveals a problem/bug for VM because with this constant folding we will have:

let %1 = fn(p0, p1) {
  add(p0, p1)
}
%2 = (constant_1, constant_2)
%3 = (tensor1,)
invoke_tvm_op(%1, %2, %3)

Then compiler would get stuck here:

https://github.com/apache/incubator-tvm/blob/c936a81dab2b4b4b595d02153a6654b9d4e09cd5/src/relay/backend/vm/compiler.cc#L463

The problem reported by Luis is actually the same, which actually wasn't a bug introduced by the mentioned PR, but it would reveal this problem.