Fix subnet counting and re-enable check for multiple onnxifi ops in AOT (#62033)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/62033
Count the number of onnxifi ops rather than just number of subnets, since when the subnet size < min_ops, it isn't turned into an onnxifi op.
Test Plan:
Runs which ran into the "Did not find a partition with an SLS node" error now report "multiple onnxifi ops found"
From https://fb.workplace.com/groups/527892364588452/permalink/807802049930814/:
```
buck run mode/opt-clang -c python.package_style=inplace sigrid/predictor/scripts:rerun_aot -- --manifold_url="https://manifold.facebook.net/v0/read/tree/2021-06-30/onnxifi_caffe2_net_aot_input_arguments_01-55-32_711d9476?bucketName=dper3_job_meta&apiKey=dper3_job_meta-key&timeoutMsec=5000&withPayload=1"
```
Reran some failures from last week which now pass AOT:
From https://fb.workplace.com/groups/527892364588452/permalink/807802049930814/,
https://fb.workplace.com/groups/243933520351820/permalink/572715897473579/
```
buck run mode/opt-clang -c python.package_style=inplace sigrid/predictor/scripts:rerun_aot -- --manifold_url="https://manifold.facebook.net/v0/read/tree/2021-07-09/onnxifi_caffe2_net_aot_input_arguments_05-31-08_ef5393a6?bucketName=dper3_job_meta&apiKey=dper3_job_meta-key&timeoutMsec=5000&withPayload=1"
```
```
buck run mode/opt-clang -c python.package_style=inplace sigrid/predictor/scripts:rerun_aot -- --manifold_url="https://manifold.facebook.net/v0/read/tree/2021-07-12/onnxifi_caffe2_net_aot_input_arguments_14-44-34_cfdf3053?bucketName=dper3_job_meta&apiKey=dper3_job_meta-key&timeoutMsec=5000&withPayload=1"
```
```
buck run mode/opt-clang -c python.package_style=inplace sigrid/predictor/scripts:rerun_aot -- --manifold_url="https://manifold.facebook.net/v0/read/tree/2021-07-13/onnxifi_caffe2_net_aot_input_arguments_04-03-30_162e7e53?bucketName=dper3_job_meta&apiKey=dper3_job_meta-key&timeoutMsec=5000&withPayload=1"
```
Reviewed By: khabinov
Differential Revision: D29796893
fbshipit-source-id: e9de7529ef86745207d41643d0fbe932fa166437
