Add a tf.contrib.image.translate function (#12306)

* Add a tf.contrib.image.translate function

* Remove redundant checks from tf.contrib.image.translate.

* Add translate and translations_to_projective_transforms to the docstring.

* Fix lint errors for tf.contrib.image.translate

* Add name_scopes in image_ops.

Indicate in the docstrings when the static shape of the arguments must
have a known rank.

* Fix pyformat's weird docstring indentation.

* tf.name_scope -> ops.name_scope

* Move the test session inside the _DTYPES loop.

* Use the default_name arg of name_scope.

* Check for ndims == None

* Fix translate docstring and add a comment.

* s/vector/matrix/ for the multiple translations.