[DistGB] modify partition.py to save FusedCSCSamplingGraph directly

[CfromBU]

change partition.py file

[dgl-bot]

To trigger regression tests:

• @dgl-bot run [instance-type] [which tests] [compare-with-branch];
  For example: @dgl-bot run g4dn.4xlarge all dmlc/master or @dgl-bot run c5.9xlarge kernel,api dmlc/master

[dgl-bot]

Commit ID: fb26900e18e5d45da064184a99b73770ea80c411

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[CfromBU]

order:
#7724 partition test case
#7727 partition
#7728 dist partition
#7690 dist partition test case

[Rhett-Ying]

this PR needs to rebase on #7724

[dgl-bot]

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