[Relay] Keep fixed dim when unifying dynamic shape

[lixiaoquan]

For this function:

fn (%True: Tensor[(?, 1), float32], %False: Tensor[(?, ?), float32]) {
  free_var %f: fn () -> bool
  %0 = %f();
  if (%0) {
    %True
  } else {
    %False
  }
}

Original type inference result:

fn (%True: Tensor[(?, ?), float32], %False: Tensor[(?, ?), float32], %f: fn () -> bool) -> Tensor[(?, ?), float32] {
  %0 = %f() /* ty=bool */;
  if (%0) {
    %True
  } else {
    %False
  }
}

Type inference result with this patch, which keeps the fixed dim

fn (%True: Tensor[(?, 1), float32], %False: Tensor[(?, 1), float32], %f: fn () -> bool) -> Tensor[(?, 1), float32] {
  %0 = %f() /* ty=bool */;
  if (%0) {
    %True
  } else {
    %False
  }
}

cc @icemelon9 @kevinthesun @zhiics Could you please review?

[tqchen]

@icemelon9 please manage this PR

[MarisaKirisame]

LGTM. @jroesch please take a look to make sure it is good. I am confused about the semantic of Any.