Add unit tests for potri and potrf backward and check output shape.

[josephevans]

Add unit tests for backward linalg_potri and linalg_potrf operators. Verify output shape.

Test results (with modified function names):

python3 -m pytest --verbose tests/nightly/test_large_array.py::test_potrf tests/nightly/test_large_array.py::test_potri
=========================================================================== test session starts ============================================================================
platform linux -- Python 3.7.7, pytest-5.4.1, py-1.8.1, pluggy-0.13.1 -- /home/ubuntu/anaconda3/bin/python3
cachedir: .pytest_cache
hypothesis profile 'default' -> database=DirectoryBasedExampleDatabase('/home/ubuntu/incubator-mxnet/.hypothesis/examples')
rootdir: /home/ubuntu/incubator-mxnet, inifile: pytest.ini
plugins: remotedata-0.3.2, openfiles-0.4.0, astropy-header-0.1.2, timeout-1.4.1, hypothesis-5.8.3, arraydiff-0.3, doctestplus-0.5.0
timeout: 1200.0s
timeout method: signal
timeout func_only: False
collected 2 items

tests/nightly/test_large_array.py::test_potrf PASSED                                                                                                                 [ 50%]
tests/nightly/test_large_array.py::test_potri PASSED                                                                                                                 [100%]

============================================================================ 2 passed in 1654.43s (0:27:34)  ============================================================================

[mxnet-bot]

Hey @josephevans , Thanks for submitting the PR
All tests are already queued to run once. If tests fail, you can trigger one or more tests again with the following commands:

• To trigger all jobs: @mxnet-bot run ci [all]
• To trigger specific jobs: @mxnet-bot run ci [job1, job2]

---

CI supported jobs: [windows-gpu, miscellaneous, website, sanity, edge, windows-cpu, unix-gpu, unix-cpu, clang, centos-gpu, centos-cpu]

---

Note:
Only following 3 categories can trigger CI :PR Author, MXNet Committer, Jenkins Admin.
All CI tests must pass before the PR can be merged.

[josephevans]

@mxnet-bot run ci [unix-gpu]

[mxnet-bot]

Jenkins CI successfully triggered : [unix-gpu]

[josephevans]

@mxnet-bot run ci [unix-gpu]

[mxnet-bot]

Jenkins CI successfully triggered : [unix-gpu]

[Zha0q1]

LGTM

[ChaiBapchya]

LGTM! Thanks!

[josephevans]

@mxnet-bot run ci [unix-gpu, unix-cpu]

[mxnet-bot]

Jenkins CI successfully triggered : [unix-cpu, unix-gpu]

[josephevans]

@mxnet-bot run ci [unix-gpu]

[mxnet-bot]

Jenkins CI successfully triggered : [unix-gpu]

[josephevans]

@mxnet-bot run ci [unix-gpu]

[josephevans]

@mxnet-bot run ci [unix-gpu]

[mxnet-bot]

Jenkins CI successfully triggered : [unix-gpu]

[josephevans]

@mxnet-bot run ci [unix-gpu]

[mxnet-bot]

Jenkins CI successfully triggered : [unix-gpu]