[RELAY] Support Random Number Generator

[tqchen]

This is a consolidated thread of relay's PRNG support. PRNG are usually considered as stateful, as a result, we will need to support the following features

• Basic block normal form (allowing reading of ref in the begining of the basic block and write to write in the end)
• Support reference in relay vm and AOT
• Formalize the PRNG in the form of the ref and stateful update
• Optionally: convert the stateful update into monad form where we explicitly pass prng state

[tqchen]

cc @altanh @junrushao1994 @jroesch @antinucleon