Add python unittest target to `caffe2/test/TARGETS` (#42766)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/42766
**Summary**
Some python tests are missing in `caffe2/test/TARGETS`, add them to be more comprehension.
According to [run_test.py](https://github.com/pytorch/pytorch/blob/master/test/run_test.py#L125), some tests are slower. Slow tests are added as independent targets and others are put together into one `others` target. The reason is because we want to reduce overhead, especially for code covarge collection. Tests in one target can be run as a bundle, and then coverage can be collected together. Typically coverage collection procedure is time-expensive, so this helps us save time.
Test Plan:
Run all the new test targets locally in dev server and record the time they cost.
**Statistics**
```
# jit target
real 33m7.694s
user 653m1.181s
sys 58m14.160s
--------- Compare to Initial Jit Target runtime: ----------------
real 32m13.057s
user 613m52.843s
sys 54m58.678s
```
```
# others target
real 9m2.920s
user 164m21.927s
sys 12m54.840s
```
```
# serialization target
real 4m21.090s
user 23m33.501s
sys 1m53.308s
```
```
# tensorexpr
real 11m28.187s
user 33m36.420s
sys 1m15.925s
```
```
# type target
real 3m36.197s
user 51m47.912s
sys 4m14.149s
```
Reviewed By: malfet
Differential Revision: D22979219
fbshipit-source-id: 12a30839bb76a64871359bc024e4bff670c5ca8b
