[Feature] QM9Edge Dataset Support

[hengruizhang98]

Description

QM9Edge Dataset Support. #2567
@mufeili

Checklist

Please feel free to remove inapplicable items for your PR.

• The PR title starts with [$CATEGORY] (such as [NN], [Model], [Doc], [Feature]])
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage
• Code is well-documented
• To my best knowledge, examples are either not affected by this change,
  or have been fixed to be compatible with this change
• Related issue is referred in this PR

[mufeili]

I still find this confusing. If a user has not installed rdkit, then this class will use the preprocessed dataset. Then when will a user ever want to preprocess from scratch and install rdkit for doing so? If you just want to display the details of preprocessing as @BarclayII suggested, then just always let users preprocess from scratch and save the preprocessed dataset to the local disk.

[mufeili]

You also need to update this file to update the documentation.

[mufeili]

I talked to Minjie. He suggested moving the code for preprocessing to a gist file and leave a link to the gist file.

[hengruizhang98]

I talked to Minjie. He suggested moving the code for preprocessing to a gist file and leave a link to the gist file.

Now the preprocessing part has been moved to : https://gist.github.com/hengruizhang98/a2da30213b2356fff18b25385c9d3cd2

[mufeili]

LGTM.