__init__.py missing in decoder only gin folder

`t5x/t5x/examples/decoder_only/models/__init__.py` should be added to avoid gin error coming from the following:

```
import gin
config = gin.config.parse_config_file("t5x/t5x/examples/decoder_only/models/xl.gin")
```

Since example `base_wmt_from_scratch.gin` relies on `base.gin` this issue might have been missed.

Add the missing t5x/t5x/examples/decoder_only/models/__init__.py file. Start by running the gin.config.parse_config_file example with t5x/t5x/examples/decoder_only/models/xl.gin, and compare with the existing base_wmt_from_scratch.gin and base.gin relationship. Done means the configuration parses without the reported gin error.

Written by the indexing model from the issue text.