Remove python boolean check from adjust_gamma (#14555)

* Update image_ops_impl.py

* Update image_ops_impl.py

* Update image_ops_impl.py

Added tensor not tensor based switch, to determine whether or not to use the tensor based assert or python boolean assert.

* Minor fixes

Check that gamma is a tensor, not image.
Update comment per style and correctness.
Print out gamma value in static error check.

* Fixed bugs in gamma implementation, switch to using _assert for tensor and python float assertion. Added unit tests.

* Use TF dtype for TF constant

* Fix style

* Fix typo in previous commit

* Update image_ops_test.py