Enable package import from torchbenchmark and torchbenchmark.models.

[xuzhao9]

More intuitive module import when installed as a package.
This PR adds LazyImport to all torchbenchmark models so that they will be loaded only when being used.

Test plan:

Run the following commands:

python install.py
pip install -e .
$ python -c "import torchbenchmark.models; print(torchbenchmark.models.densenet121.Model)"
<class 'torchbenchmark.models.densenet121.Model'>
$ python -c "import torchbenchmark; print(torchbenchmark.models.densenet121.Model)"
<class 'torchbenchmark.models.densenet121.Model'>

Fixes #2193

[facebook-github-bot]

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[facebook-github-bot]

@xuzhao9 merged this pull request in 3bb81e5.