Add support of `axis` for `tf.unique` (#12952)

* Convert unique implementation to be compatible with axis

This commit made the conversion so that unique could
be extended with axis. A custom unordered_map is used.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Add support of `axis` for `tf.unique`

This commit adds support of `axis` for `tf.unique`.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Add test cases of `axis` support for `tf.unique`.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Address review feedback.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Change forward_input_or_allocate_output to allocate_output to fix

the following failures:
```
//tensorflow/python/debug:analyzer_cli_test
//tensorflow/python/debug:session_debug_file_test
//tensorflow/python/feature_column:feature_column_test
```

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>