…nblock production (pytorch#167443)

Summary:

pytorch#166609 updates `node.is_impure` to consider a submodule as impure if submodule contains impure node. Prior to pytorch#166609, we always consider a submodule as pure, regardless of what ops contained inside. 


pytorch#166609 fixed that impurity gap, and in turn changed `graph.eliminate_dead_code()` function behavior, which does not eliminate nodes with side effects, see [pytorch documentation](https://docs.pytorch.org/docs/stable/fx.html#torch.fx.Graph.eliminate_dead_code)
> Remove all dead code from the graph, based on each node’s number of users, and whether the nodes have any side effects. 

As a result, dead code elimination may no longer remove *some* unused submodules if they have impure ops. While this is logically correct, some customers rely on the dead code elimination to eliminate impure submodules.

Due to such production environment constraints, we have to revert pytorch#166609 and move the side-effectful submodule check logic to `const_fold.py`, which will correctly **not** const-fold a submodule that contains impure ops.

NOTE other call sites that use `node.is_impure()` to make decisions are still incorrectly eliminating impure submodules, but we can't safely change that today.


## This pr
- move `_subgraph_has_impure_op` into `fx/experimental/const_fold.py`, check and prevent const-folding an impure submodule 
- added a note in `node.is_impure` to highlight the incorrect behavior and context in case people go looking in the future.

Test Plan: run test_fx_const_fold and all tests pass

Differential Revision: D86641994

… unblock production (pytorch#167443)

Summary:
pytorch#166609 updates `node.is_impure` to consider a submodule as impure if submodule contains impure node. This in turn changes `graph.eliminate_dead_code()` function behavior, which does not eliminate nodes with side effects, see [pytorch documentation](https://docs.pytorch.org/docs/stable/fx.html#torch.fx.Graph.eliminate_dead_code)
> Remove all dead code from the graph, based on each node’s number of users, and whether the nodes have any side effects.

While this is correct that a submodule containing side-effectful ops is side-effectful and should not be dead code eliminated, some customers rely on the dead code elimination to eliminate submodules that contain impure ops which is the behavior before pytorch#166609 fix.

Due to production environment constraints, we have to revert pytorch#166609 and move the side-effectful submodule check logic to `const_fold.py`, which will correctly **not** const-fold a submodule that contains impure ops.

NOTE other call sites that use `node.is_impure()` to make decisions are still incorrectly eliminating side-effectful submodules, but we can't safely change that today.

## This pr
- move `_subgraph_has_impure_op` into `fx/experimental/const_fold.py`, check and prevent const-folding an impure submodule
- added a note in `node.is_impure` to highlight the incorrect behavior and context in case people go looking in the future.

Test Plan: run test_fx_const_fold and all tests pass

Differential Revision: D86641994

Pull Request resolved: pytorch#167443
Approved by: https://github.com/jfix71