[Dist] format dtypes when loading graph in server

[Rhett-Ying]

Description

fix for #4222

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 the best of my 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
• If the PR is for a new model/paper, I've updated the example index here.

Changes

[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: ff45e4129499d1d6c1611ba3d97b7cb46bf0407f

Build ID: 1

Status: ✅ CI test succeeded

Report path: link

Full logs path: link

[dgl-bot]

Commit ID: 878bb9f6f9740a785a82e5554dfb547d78eebbe3

Build ID: 2

Status: ✅ CI test succeeded

Report path: link

Full logs path: link

[dgl-bot]

Commit ID: dc00a0a3801e37b41c3098e8a1ed7b95a7d235f6

Build ID: 3

Status: ✅ CI test succeeded

Report path: link

Full logs path: link

[dgl-bot]

Commit ID: de384e834c2f954646627ab83a5fb1acb9587c0e

Build ID: 4

Status: ✅ CI test succeeded

Report path: link

Full logs path: link

[dgl-bot]

Commit ID: cc5ef23

Build ID: 5

Status: ✅ CI test succeeded

Report path: link

Full logs path: link