This commit adds the output file name to the log message when the
conversion is completed.

The motivation for this change is that when `--outfile` option is not
specified it migth not be obvious where the output file is written.

With this change the output of running the script will be something like
the following:
```console
INFO:hf-to-gguf:Model successfully exported to models/gemma-2-9b-it.gguf.
```

Signed-off-by: Daniel Bevenius <daniel.bevenius@gmail.com>

Updates the output of to support printing the directory if the output is
split into multiple files. Also the output file name is now retrieved
from the model_instance object.

Signed-off-by: Daniel Bevenius <daniel.bevenius@gmail.com>

Use parent attribute of Path object and string interpolation.

Signed-off-by: Daniel Bevenius <daniel.bevenius@gmail.com>

Use os.sep instead of hardcoding the path separator.

Signed-off-by: Daniel Bevenius <daniel.bevenius@gmail.com>