[Fori_loop|While_loop] Enable while_loop/fori_loop, Add linear/MNIST test case

[ManfeiBai]

• Modify BuildForiLoop for while_loop/fori_loop
• Modify test code for while_loop/fori_loop with simple version
• Add linear_layer_model and MNIST_model(without BN layer) test case

[JackCaoG]

can we init the l_out inside the mnist forward and returns it? This way the only additional output we provide is the count.

[ManfeiBai]

thanks, good idea, due to hard to gain result before finish one run, so current plan would be:
run body_fn with pure python while, get result, then use this result as l_out

before implementation, did quick benchmark for this idea with code:

- iteri = 3: 
  while_loop(3.675 s), pure while(3.107 s), pure_while_then_while_loop_plan(3.724 s)
- iteri = 10: 
  while_loop(3.670s), pure while(3.119s), pure_while_then_while_loop_plan(3.932s)
- iteri = 10: 
  while_loop(3.744 s), pure while(3.110 s), pure_while_then_while_loop_plan(3.749 s)
- iteri = 10: 
  while_loop(3.878 s), pure while(7.053 s), pure_while_then_while_loop_plan(3.809 s)

so this plan looks like worth to try

[JackCaoG]

only review the test part, left some comments

[JackCaoG]

@ManfeiBai give me a ping when CI is green

[ManfeiBai]

fork PR failed, track in #7157

[ManfeiBai]

thanks for review and comments, since CI failed to start, track test result in #7157