[skip ci] Add PULL_REQUEST_TEMPLATE.md (#5340)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,11 @@
+## What?
+
+<!-- Please write what you changed. -->
+
+## Why?
+
+<!-- Please write why you changed. -->
+
+## See also
+
+<!-- Write additional information if necessary (e.g., referecne, related PRs or Issues). -->

CONTRIBUTING.md

@@ -140,7 +140,7 @@ have the format `def test_yyy(...)`.  [Pytest](https://docs.pytest.org/en/latest
 Technically, a test file should only cover methods from one file (e.g.: `test_transformer_utils.py` to test `transformer_utils.py`).
 - To monitor the test coverage and avoid the overlapping test, we recommend using  `pytest --cov-report term-missing <test_file|dir>`
 to highlight covered and missed lines. For more details, please refer to [coverage-test](https://pytest-cov.readthedocs.io/en/latest/readme.html).
-- We limit test running time to 2.0 seconds (see: [pytest-timeouts](https://pypi.org/project/pytest-timeouts/)) for each trial. As such, we recommend using small model parameters and avoiding dynamic imports, file access, and unnecessary loops. 
+- We limit test running time to 2.0 seconds (see: [pytest-timeouts](https://pypi.org/project/pytest-timeouts/)) for each trial. As such, we recommend using small model parameters and avoiding dynamic imports, file access, and unnecessary loops.
 If a unit test needs more running time, you can annotate your test with `@pytest.mark.execution_timeout(sec)`.
 - For test initialization (parameters, modules, etc), you can use pytest fixtures. Refer to  [pytest fixtures](https://docs.pytest.org/en/latest/fixture.html#using-fixtures-from-classes-modules-or-projects) for more information.