Undefined name: 'value_and_grad' in ./tests/api_test.py
[flake8](http://flake8.pycqa.org) testing of https://github.com/google/jax on Python 3.7.1
$ __flake8 . --count --select=E901,E999,F821,F822,F823 --show-source --statistics__
```
./tests/api_test.py:54:12: F821 undefined name 'value_and_grad'
assert value_and_grad(f)(1.0, 1.0, 1.0, flag=True) == (y, 1.0)
^
./tests/api_test.py:55:12: F821 undefined name 'value_and_grad'
assert value_and_grad(f, argnums=1)(1.0, 1.0, 1.0, flag=True) == (y, 2.0)
^
./tests/api_test.py:56:12: F821 undefined name 'value_and_grad'
assert value_and_grad(f, argnums=(2, 0))(1.0, 1.0, 1.0, flag=True) == (y, (3.0, 1.0))
^
3 F821 undefined name 'value_and_grad'
3
```
__E901,E999,F821,F822,F823__ are the "_showstopper_" [flake8](http://flake8.pycqa.org) issues that can halt the runtime with a SyntaxError, NameError, etc. These 5 are different from most other flake8 issues which are merely "style violations" -- useful for readability but they do not effect runtime safety.
* F821: undefined name `name`
* F822: undefined name `name` in `__all__`
* F823: local variable name referenced before assignment
* E901: SyntaxError or IndentationError
* E999: SyntaxError -- failed to compile a file into an Abstract Syntax Tree
