Fix data path and termination in headless rendering python example

[ShreyanshDarshan]

This change is 

[update-docs[bot]]

Thanks for submitting this pull request! The maintainers of this repository would appreciate if you could update the CHANGELOG.md based on your changes.

[reyanshsolis]

TODO: Fix a design for storing example outputs if any. Suggestion: open3d_data / example_results / example_name /

This will also be followed later for the reconstruction system, where there are multiple outputs and storing output in the input dataset dir. does not look clean (we should not write anything to dataset dir).

[reyanshsolis]

let's fix this up quickly and merge the pr

[reyanshsolis]

• added review changes.
• added data.output_dir [like data.extract_dir], to return the absolute path to an output dir corresponding to the dataset to store results. i.e. open3d_data/output/dataset_name/. Currently, the reconstruction systems save the output in the dataset folder itself (by default design that will be the open3d_data/extract/dataset_name/ folder for us), which does not seem to be a very clean design to me. Would like to check with @yxlao regarding the design.