[inductor] Add torch.cond support to JIT Inductor (#119759)
Summary: `torch.cond` is already supported in Dynamo and Export: the `true_fn` and `false_fn` subgraphs are traced as child fx graphs of the main graph and passed to the `torch.cond` higher-order operator in the fx graph. However, this breaks in Inductor, as the latter doesn't have the ways of dealing with child fx subgraphs and properly lowering and codegen-ing them.
In this PR, we add `torch.cond` support in Inductor. This is achieved by adding subgraph lowering and codegen-ing infrastructure as well as new `Conditional` IR node type weaving the parent graph with the true and false child subgraphs.
Here we only implement `torch.cond` support in JIT Inductor (Python wrapper codegen). The implementation in AOT Inductor (C++ wrapper codegen), including ABI-compatibility mode, will follow.
Test Plan:
```
$ python test/inductor/test_control_flow.py
...
----------------------------------------------------------------------
Ran 24 tests in 86.790s
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/119759
Approved by: https://github.com/jansel, https://github.com/eellison
