Update core/download docstrings

[sharanramjee]

Description

Updated docstrings and fixed old pylint errors in core/download files.

[Conchylicultor]

Thank you for trying to make our code to looks better.

However, my feeling is that we shouldn't spend too much time on those cosmetic changes when there are much more important / impactful tasks to do.

For instance, changing:

def asdict(self) -> Dict[str, Any]:
"""Returns the dict representation of the dataclass."""

To:

def asdict(self) -> Dict[str, Any]:
"""Returns the dict representation of the dataclass.

Returns:
  Dict representation of the dataclass.
"""

Doesn't add really any value (neither for the developer, nor for the final user).

When you have a limited number of time, you should try focussing on things which really matter. For instance adding new features. It would also be much more interesting for you.