Do not pin docstring-parser to a fixed version

[moeslami]

Description

We request to please do not pin the docstring-parser to a fixed version such as 0.8.1 as it caused conflicts with other python packages we consume along with torchx. Thanks.

Motivation/Background

Pinning docstring-parser to a fixed version like docstring-parser==0.8.1 causes dependency conflicts when installing some other python packages.

Detailed Proposal

Make torchx depend on docstring-parser>=0.8.1,<=0.15.0 or something similar

Alternatives

Additional context/links

[kiukchung]

Thanks for the suggestion. #772 should do it.