Change module scope

[Jacob-Chmura]

Purpose

The purpose of this PR is to move DGraph and DGBatch into top level scope. This enables imports like:

from tgm import DGraph, DGBatch

which seems better (in the same was DataFrame and tensors are in top level pandas and pytorch scope, for instance).

Key Changes

• Move DGraph and DGBatch into top level scope
• Changed all examples to use the new imports
• Removed pandas import for type check in DGraph (since user may not have it). This was caught by the docker image build smoke test

Things to keep in mind

• Kept hooks in their own module as they will likely soon be split into separate files
• Kept DGDataLoader in separate module similar to torch
• DGData is left as is since its mostly private (users can just construct DGraphs, not worry about internal IO)
• TimeDeltaDG is on the fence, but I will likely change DGraph constructor to take a string so that user never has to explicitly construct a time delta (Let DGraph constructor take TimeDelta as a string #72):

DGraph('tgbl-wiki', 's') # instead of `DGraph('tgbl-wiki', TimeDeltaDG('s'))

Relevant Prs

Close #68

[shenyangHuang]

I agree with this change, much more intuitive

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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