[Relay][Interpreter] Build self referential closure only when letrec is defined

[hlu1]

Self referential closure creates memory leaks. We can avoid this memory leak in cases where recursive functions are not used. As discussed in #3423, garbage collection or weak pointer is needed to fix the memory leak in the recursive case.

Before:

==3058763== LEAK SUMMARY:
==3058763==    definitely lost: 48 bytes in 1 blocks
==3058763==    indirectly lost: 128 bytes in 3 blocks
==3058763==      possibly lost: 82,062 bytes in 1,667 blocks
==3058763==    still reachable: 261,640 bytes in 4,301 blocks
==3058763==                       of which reachable via heuristic:
==3058763==                         stdstring          : 50,132 bytes in 1,037 blocks
==3058763==         suppressed: 0 bytes in 0 blocks

After:

==2878348== LEAK SUMMARY:
==2878348==    definitely lost: 0 bytes in 0 blocks
==2878348==    indirectly lost: 0 bytes in 0 blocks
==2878348==      possibly lost: 82,062 bytes in 1,667 blocks
==2878348==    still reachable: 261,640 bytes in 4,301 blocks
==2878348==                       of which reachable via heuristic:
==2878348==                         stdstring          : 50,132 bytes in 1,037 blocks
==2878348==         suppressed: 0 bytes in 0 blocks

@jroesch, @MarisaKirisame please review.

[MarisaKirisame]

This fix it for this case. I am just in general unfavor of case by case fix, as it will always be incomplete (by rice theorem). I think some cycle detection on top of ref counting will be best and I will talk with @jroesch .

[hlu1]

I totally agree. This does unblock us from deployment while you guys work on a more proper fix.

[tqchen]

thanks, @hlu1 @MarisaKirisame , this PR is now merged.