[TypeChecker][Relay] Issue with return type not being properly inferred.

[jroesch]

cc @hypercubestart after I land #6274 there is an issue where omitting a return type now causes inference to fail, when you work on mutual recursion can you look into this?

 // (@jroesch): if we leave off the return type this doesn't work
/*
 * Computes the sum of a list of integer scalars.
 */
 // (@jroesch): if we leave off the return type this doesn't work
def @sum(%xs: List[Tensor[(), int32]]) -> int32 {
  let %add_f = fn(%x: Tensor[(), int32], %y: Tensor[(), int32]) -> Tensor[(), int32] {
    %x + %y
  };
  @foldl(%add_f, 0, %xs)
}

/*
 * Computes the size of a tree.
 */
def @size[A](%t: Tree[A]) -> Tensor[(), int32] {
  match(%t) {
    Rose(_, %sub_trees) => {
      1 + @sum(@map(@size, %sub_trees))
    },
  }
}

[jroesch]

https://github.com/jroesch/tvm/tree/tyck-err

[jroesch]

This also type checks:

/*
 * Reverses a list.
 */
def @rev[A](%xs: List[A]) -> List[A] {
  @foldl(@flip(fn (%h: A, %t: List[A]) { Cons(%h, %t) }), Nil, %xs)
}

This one doesn't type check.

/*
 * Reverses a list.
 */
def @rev[A](%xs: List[A]) -> List[A] {
  @foldl(@flip(Cons), Nil, %xs)
}

[tqchen]

@jroesch please update to see if the issue can be closed