[caffe2] Check for number of created subnets and optionally throw an error (#57366)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/57366
We often get error messages such as
```
Model failed AOT (glow ahead-of-time compilation) with exception: Error during AOT optimization (non-provisioned addNetwork):
Non-recoverable device error when adding network:
Error code: PARTITIONER_ERROR
Error message: Did not find a partition with an SLS node
Error return stack:
--------------------------------------------------------------------------------
glow/glow/lib/Partitioner/Partitioner.cpp:1244
--------------------------------------------------------------------------------
glow/glow/lib/Runtime/HostManager/HostManager.cpp:375
--------------------------------------------------------------------------------
```
This makes the error message more clear by checking for the number of OnnixifiOp created before going into Glow. The check is enabled with the `verify_only_single_subnet` flag, and is disabled by default.
Test Plan: Unit tests pass
Reviewed By: khabinov
Differential Revision: D28097674
fbshipit-source-id: 0eefd8f6ec1a82546b759be8e541256bf271a673
