torchx/schedulers: added support for image_type=docker

[d4l3k]

This adds support for image_type=docker. Image fetchers have been renamed to image providers as docker needs a special CLI command in order to run the docker container.

To select docker:

torchx run --scheduler local --scheduler_args image_type=docker echo.torchx --image pytorch/pytorch:latest

Test plan:

pyre
python setup.py test -s torchx.schedulers
python torchx/cli/main.py run --scheduler local --scheduler_args image_type=docker echo.torchx --image pytorch/pytorch:latest

[facebook-github-bot]

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[facebook-github-bot]

@d4l3k merged this pull request in 322a465.