[caffe2] Fix duplicate name bug in Net.AddExternalInput (#47530)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/47530
`Net.AddExternalInput` should raise if there are duplicate names. The previous code would only raise if the addition of duplicates was in separate calls, but not if it was in the same call.
Test Plan:
Added two new regression tests
```
✓ Pass: caffe2/caffe2/python:core_test - testSetInputRecordWithBlobs (caffe2.caffe2.python.core_test.TestExternalInputs) (9.622)
✓ Pass: caffe2/caffe2/python:core_test - testAddExternalInputShouldRaiseIfDuplicate (caffe2.caffe2.python.core_test.TestExternalInputs) (9.639)
✓ Pass: caffe2/caffe2/python:core_test - testSetInputRecordWithoutBlobs (caffe2.caffe2.python.core_test.TestExternalInputs) (9.883)
✓ Pass: caffe2/caffe2/python:core_test - testAddExternalInputShouldRaiseIfDuplicateInSameCall (caffe2.caffe2.python.core_test.TestExternalInputs) (10.153)
```
Test trained 2 models. No issues
f230755456
f230754926
Reviewed By: dzhulgakov
Differential Revision: D24763586
fbshipit-source-id: c87088441d76f7198f8b07508b2607aec13521ed
