Recomend pip install -r requirements in the unit testing guidelines. (#137797)

Somehow make setup-env as recomended in CONTRIBUTING.MD is not installing all dependencies require to run tests

This makes it slightly clearer when running tests.

Specific repro on my side was
```
git checkout e7679663070e3149ae7cd6e28d376d86852ce9e4
make setup-env
conda activate pytorch-deps
python test/test_utils_internal.py
```

which is what my reading of the instructions implies should be correct.

Pull Request resolved: https://github.com/pytorch/pytorch/pull/137797
Approved by: https://github.com/albanD

CONTRIBUTING.md

@@ -286,6 +286,11 @@ The following packages should be installed with either `conda` or `pip`:
 - `expecttest` and `hypothesis` - required to run tests
 - `mypy` - recommended for linting
 - `pytest` - recommended to run tests more selectively
+Running
+```
+pip install -r requirements
+```
+will install these dependencies for you.
 
 All PyTorch test suites are located in the `test` folder and start with
 `test_`. Run the entire test