[cherry-pick] [Fori_loop|While_loop] Create fori_loop.md (#6942)

[ManfeiBai]

Add doc for fori_loop/while_loop and add simple user guide for simple test case

[yeounoh]

LGTM

[yeounoh]

If not all the fori_loop PRs are in r2.3, then maybe we shouldn't back-port this... I will leave it up to you and Siyuan. @ManfeiBai @lsy323

[JackCaoG]

If user tried to run the code in the doc with 2.3 whl, would it work? if not we should not backport.

[ManfeiBai]

If user tried to run the code in the doc with 2.3 whl, would it work? if not we should not backport.

we have while_loop and fori_loop in this PR at the same time, and we only include PR for while_loop in 2.3,

so I could update this PR to only include while_loop version, and delete fori_loop part and push local change to this branch again

[ManfeiBai]

If user tried to run the code in the doc with 2.3 whl, would it work? if not we should not backport.

Thanks, tried with 2.3RC12 wheel(torch and torch_xla) and passed test case mentioned in md file, also passed tests in test/test_fori_loop_with_while_loop_simple_add_dispatch_in_torch.py

[ManfeiBai]

If not all the fori_loop PRs are in r2.3, then maybe we shouldn't back-port this... I will leave it up to you and Siyuan. @ManfeiBai @lsy323

Thanks, @lsy323, I updated the doc to only mention feature included in 2.3, do we wanna backport this doc change?

[lsy323]

@ManfeiBai Backport docs sounds good to me.