Change mutable default arguments to None

[bwroblew]

Reading PyG code I accidently saw that class and noticed that it uses mutable default arguments what is generally considered a bad practice and can lead to a lot of problems. I believe it's not a desired behaviour (am I wrong?).

Problems can occur when the class is extended in the future such that it changes the value of any of those lists, the default value for new instances of this class would also be affected. The other possibility is that user changes any of the follow_batch, exclude_keys fields (those doesn't start with an underscore, so it's kinda possible) after the object is initialized, what's also going to change the default value for new instances.

[codecov]

Codecov Report

Merging #6376 (279e948) into master (8e4f967) will not change coverage.
The diff coverage is n/a.

❗ Current head 279e948 differs from pull request most recent head 9de1c8b. Consider uploading reports for the commit 9de1c8b to get more accurate results

@@           Coverage Diff           @@
##           master    #6376   +/-   ##
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  Coverage   84.52%   84.52%           
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  Files         393      393           
  Lines       21563    21563           
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  Hits        18227    18227           
  Misses       3336     3336           

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[rusty1s]

Thank you!