[fx] PassManager changes (#80531)
PassManager is a class used to run multiple passes on a given graph module.
Class Attributes
* `passes: List[Callable]`: A list of callable passes
* `constraints: List[Callable]`: A list of constraints
* `run_checks_after_each_pass`: Flag for running checks each pass
Class Methods:
* `__call__(graph_module: DispatchGraphModule)`:
* Runs the passes based on the list of passes until the graph stops changes, or until `steps` number of times.
* Each time a pass is run, it will check that the graph module still maintains the required invariants by calling `check()` and will lint the graph to check that it’s well formed if the flag `run_checks_after_each_pass` is set.
* `check(graph_module: DispatchGraphModule)`: Runs various checks on the given graph module to make sure that it contains the needed data for passes
* `add_check(check: Callable)`: Adds the `check` function to the given pass manager instance
* `add_constraint(constraint: Callable)`: Adds a constraint to the current list of constraints
We can create a PassManager and run it by doing:
```
PassManager(passes=[pass1, pass2])(graph_module)
```
Differential Revision: [D37523159](https://our.internmc.facebook.com/intern/diff/D37523159)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/80531
Approved by: https://github.com/SherlockNoMad
