Fix exception chaining

[akihironitta]

I recently went over PyTorch and Detectron2, suggesting a fix in the way that Python 3's exception chaining is used.

As described in detail in this article, exception chaining (PEP 3134) can be used to make exceptions more user-friendly, and in that case, the syntax raise new_exception from old_exception needs to be used.

When raise .. from .. is used, there will be a line saying The above exception was the direct cause of the following exception between tracebacks. However, when implicitly chaining exceptions (meaning when raise .. from .. is not used), the message will be During handling of the above exception, another exception occurred which can confuse users.

Specifically, the following should be used in order to chain exceptions:

try:
    something_which_raises_OldError
except OldError as e:
    raise NewError("A more user-friendly exception message.") from e 

instead of:

try:
    something_which_raises_OldError
except OldError:
    raise NewError("A more user-friendly exception message.")

One example which needs to be fixed is:
https://github.com/PyTorchLightning/pytorch-lightning/blob/3d76f604bdb97d66dd1248540ea088fb18c06a03/pytorch_lightning/utilities/parsing.py#L159-L162

If this suggestion sounds good and reasonable, I'd be happy to create a PR! Let me know your thoughts on this!

[github-actions]

Hi! thanks for your contribution!, great first issue!

[awaelchli]

oh! I just learned something new about Python exceptions. Awesome!
So if I understand correctly, this change will make the exceptions more user friendly, pointing to where it originated from.
Do we keep the bugfix label? It sounds more like an enhancement

Related: #2266

[akihironitta]

Thanks @edenlightning for relabelling the issue! I didn't know Hacktoberfest at all!

@awaelchli Thank you for your quick response!
Yes, you're right. Since this issue is not critical, can I work on this in October as a PR for Hacktoberfest?

[rohitgr7]

@akihironitta assigned you to this issue. Yeah, you can do this in October.

[akihironitta]

@awaelchli @rohitgr7 I just made #3750 ready for review! Can you have a look at the changes?