[caffe2] Add unittests for schema.Field init (#47512)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/47512
I deleted the last line of `__init__` -- `self._field_offsets.append(offset)` -- and the unittests didn't fail.
So this diff is to improve test coverage.
Test Plan:
```
✓ Pass: caffe2/caffe2/python:schema_test - testInitShouldSetEmptyParent (caffe2.caffe2.python.schema_test.TestField) (8.225)
✓ Pass: caffe2/caffe2/python:schema_test - testInitShouldSetFieldOffsetsIfNoChildren (caffe2.caffe2.python.schema_test.TestField) (8.339)
✓ Pass: caffe2/caffe2/python:schema_test - testInitShouldSetFieldOffsets (caffe2.caffe2.python.schema_test.TestField) (8.381)
```
Reviewed By: dzhulgakov
Differential Revision: D24767188
fbshipit-source-id: b6ce8cc96ecc61768b55360e0238f7317a2f18ea
