[DEVELOPMENT] Control flow and LSTM demo milestones

[YuchenJin]

• [x] Lambda lift pass
• [x] Support closure
• [x] Add relax.while_loop similar to the current relay.while_loop (functional through recursion)
• [x] TensorArray support: enable opaque Object type and implement TensorArrayRead/TensorArrayWrite PackedFuncs.
• [x] E2E LSTM demo
• [ ] Consider adding while and foreach intrinsics (imperative through iteration)

[ZihengJiang]

One question, why do we need to support TensorArray for control flow? in order to store the results in the while loop?

[yongwww]

One question, why do we need to support TensorArray for control flow? in order to store the results in the while loop?

TensorArray is commonly used with control flow, for example in the TensorFlow while_loop, TensorArray is utilized to store the results, there are tons of TensorArrayRead/TensorArrayWrite operations in the loop. TensorArray support is necessary for end2end model support.