Skips docker push unless explicitly called out in the inputs for the
calculate-docker-image step.

Also skips the push if the image already exists on ECR due to our
immutable image tags

Signed-off-by: Eli Uriegas <eliuriegas@fb.com>
Pull Request resolved: https://github.com/pytorch/pytorch/pull/80402
Approved by: https://github.com/janeyx99, https://github.com/malfet